Gaming

You've already got a bunch of great suggestions, but I'll throw my two cents in too. I think of these as indies or games that have a niche audience, but some of them probably have bigger teams involved.

• Outer Wilds. It's a quiet, contemplative game about space exploration and seems unremarkable until you really start exploring and learning about the solar system you're in. If you play it, go in blind. IMO it's a perfect game.
• Pentiment. You play as a young artist visiting a Renaissance-era hamlet to work on your masterpiece. The game is gorgeous and thoughtful about the decisions the player has to make when spending time and influencing the narrative. Stayed with me long after I finished.
• Slay the Spire. Tightly-balanced deck builder rougelike. I've put untold hours into this one on maybe 3 different platforms?
• Wildermyth. Hard to describe this one--it's like an interactive fantasy tale with characters that grow and evolve in ways you won't expect, and dynamically told so no two stories are quite the same. Pretty solid combat mechanics, too.
• Undertale. Does Undertale count as relatively unknown? If so, Undertale.
• Case of the Golden Idol. Very good mystery game with a cool, novel mechanic for solving each mystery. Don't let the aesthetic put you off.
• Return of the Obra Dinn. Another incredible mystery game where you are determining what happened to the crew of a ship that sailed into port with no one aboard.
• Papers Please. Simple mechanics that really make you FEEL like you're -Spider-Man- an oppressed civilian in a fascist/totalitarian government.
• Disco Elysium. Another great mystery game, wherein part of the mystery is: what kind of person were you, and who are you now?
• Tunic. It's like old-school Zelda but with more depth and some serious twists.
• Stanley Parable. It's funny and fun.
• Tchia. It's like Breath of the Wild if Hyrule was a real place and the game devs wanted you to love it as much as they do. Fun and charming in a big way.
• Season. Hard to describe and I haven't finished it yet, but the opening stuck with me. It's about what we remember and what we choose to forget.
• Hollow Knight. Does Dark Souls arguably better than Dark Souls. I was put off by the Dark Soulsness and the aesthetic, but once I got a few hours in I was fully hooked. Another perfect game.
• Deep Rock Galactic. Great fun mining valuables and fighting alien bugs with friends.
• 13 Sentinels: Aegis Rim. A time-traveling anime story game with mecha combat.
• The Forgotten City. A time loop game where you are sent back to ancient Roman time.
• Nonary Games / Danganronpa. Both series are anime-inspired mystery games. If you want something that tries to be more cerebral and serious, Nonary is your ticket. If you want tongue-in-cheek violence, Danganronpa is the way to go.
• The Witness. A masterpiece puzzle game built on an incredibly simple core design. One of my favorite all time "a ha" moments came from this game.
• Citizen Sleeper. You are a synthetic being trying to survive on a space station and evade the megacorp that "owns" you. Your body is breaking down and you don't know anyone, so you have to take it one day at a time and do what you can to survive. Clever mechanics and a really well told narrative.

That ended up being a lot more than I intended to share, but if this convinces anyone to give any of these games a shot, I think they're in for a treat.

Dredge is pretty recent but it definitely impressed me with how it conveyed fear/paranoia in an eldritch horror setting. The fishing was also pretty fun!

• CrossCode - Phenomenal action RPG. Combat is fast and explosive, dungeons are very obviously Zelda inspired but with way more puzzles. Packed with tons and tons and tons of sidequests, many of which put unique twists on the combat system to keep you on your toes. Make sure to grab the epilogue DLC.

• FOOTSIES - Minimalist one-button fighting game, with rollback.

• OneShot - Fairly reminiscent of Undertale, if you liked that you'll probably enjoy this too. And like Undertale I don't want to say too much, take my word for it and let it surprise you.

• Petal Crash - Absolutely fantastic versus puzzler, and a perfect entry point into the genre. I wrote a very long review of how in love with this game I am, so I'm just going to link that.

• Them's Fightin' Herds - Another great fighting game, been waiting a long time for this port to bring us up to a grand total of two good fighting games on Linux. Has a lot of really cool features like a big story mode with overworld exploration, a cute lobby system with cosmetics to collect and treasure chests to fight for, a dynamic music system that reacts to the fight, and even a semi-cooperative dungeon crawler mode. Has crossplay with consoles as well. Full review.

• Ultimate Chicken Horse - Start on a nearly empty platformer map, each round everyone adds one object somewhere on the map then you all try to finish the level. Whoever finishes gets a point, plus bonus points for whoever finished first or collected coins that have been placed. Then you add another set of objects and repeat. Quickly becomes hilariously chaotic as you try to figure out how to balance screwing everyone else over while still making sure you can win, only to realize that after a few rounds you have all built a horrifying monstrosity. Has full crossplay with console versions.

• Anything by Zachtronics - A bunch of different engineering puzzle games where you have to write code or build a machine to solve problems. Once you've solved the puzzle, you can see a histogram comparing your solution to everyone else's on a few different metrics, encouraging you to go back and try to optimize it further. I recommend Opus Magnum as the best entry point.

Firewatch - recommend you wait for a sale but its a great story game that is on steam and gog and even native Linux

tabletop simulator - tons of card and board games that you can play with your friends if it exists it might be ported has native Linux

golf with your friends - great mini golf game to play with your friends similar to tower unite golf but better has native port

american/europe truck simulator - trucking game that real truckers say is the most realistic trucking game and they try to help with all issues and is even native Linux

https://store.steampowered.com/app/1641020/my_brother_lives_in_a_canyon - my brother lives in a canyon - great story game that is made by one person and it does not take long

Bloons tower defence 6 - tower defence game where you pop the bloons to defend your base

Symphony of War: The Nephilim Saga

I'll say up front that it's a niche game and isn't for everyone. This is especially true because most of its aspects outside of the gameplay are pretty unremarkable. But if this is your thing, it's really your thing.

What makes it so special is that it's core gameplay loop is a very satisfying meld of Fire Emblem and Ogre Battle. And if that statement made you perk up just by reading it, you need to go buy this game ASAP.

Now, if that description didn't immediately pique your interest, I'd check out some gameplay videos instead. Because it fills such an obscure niche, it's actually hard to know if you'll like this game just from a brief description. There's a good chance you've never played anything like this, and it will scratch an itch you never knew you had.

Everything - a zen game about everything, very chill

The Sexy Brutale - weird name, actually a fantastic time travelling LucasArts adventure style game

Neo Scavenger - ignore the graphics, this is a really in-depth turn based post-apocalyptic mini rpg where you will come to love plastic bags

Cataclysm: Dark Days Ahead isn't really indie, it is straight up Open Source and a freaking good roguelike. The DDA variant focus heavily on the realism aspect of survival, but there are forks that try to gamefy it more.

There is also Rain World, which I am addicted for 3+ months now. It is also on sale, but before you buy it, know that you are going to die A LOT, but it really isn't a big issue; it's actually funny most times lol. Great lore too.

And one that I've rooted for a long time and I think flopped hard in its release is Below, but I very much still love it.

Also, one that I played for a bit last week and the soundtrack slaps hard is APE OUT.

How did I forget about Noita? This one is a masterpiece. Crafting a wand just to fucking obliterate yourself out of existence .3 seconds later is just chef's kiss. Also hämis.

Spelunky HD

A roguelike platformer. While it lacks any meta-progression (like the similar Rogue Legacy games), the core platforming keeps me coming back again and again. The systems-based nature of the game keeps things fresh through many play-throughs.

Umurangi Generation

A photography game that's more cyberpunk than the game titled Cyberpunk. It gradually introduces you into its neon-soaked, Neon Genesis Evangelion-inspired world where humanity fights the good fight against kaiju.

Florence

A story of love, loss, and moving on.

• Ring of Pain is a great roguelike.
• Paradise Killer - Not actually sure how to describe Paradise Killer, but it's one of my favourite games.
• The Red Strings Club - Interesting, short and sweet
• Stephen's Sausage Roll - If you love puzzle games and can't find any that are hard enough for you, it might be the best game ever made. Or the most infuriating.

And then a few that maybe aren't that obscure by now, but I can't not mention them

• A Short Hike - I think possibly the greatest small indie game of all time? Certainly one of my favourites.
• Baba Is You - (slightly) less extreme puzzle game, but it's also great.
• Inscryption - Just a great game, can't say much about it without spoiling the experience

My wallet hurts, but my steam library might be able to help.

• The Dungeon Beneath is a turn based party building roguelite. Easy to understand but can get pretty challenging. Also the music slaps.

• 20 Minutes Till Dawn has a spin on the Vampire Survivors formula that I quite enjoy. Merging the crazed progression of items and swathes of enemies, with a twin stick shooter.

• Rodina was what I played when elite and nms were disappointing, and star citizen was still pretending to aim to deliver a final product. For a 1 person team, it really impressed me.

• Get to the Orange Door is a rougelite spin on Titanfall 2 in a weird cyberspace world. The biggest missing feature is a grappling hook.

• Sun Haven is a little buggy but a more rpg and combat focused Stardew Valley. I used to play a lot of rune factory 3, so I enjoyed it.

I tried to recommend some stuff I thought other people wouldn't have played, so I hope you enjoy!

Distance is an arcade racing game with a neon cyberpunk aesthetic. And the main campaign has elements of horror.

It's one of the most unique games I've played - it's almost more of a platformer than a racing game. And the main campaign is worth the asking price alone, but there's 2 other campaigns, a bunch of standalone maps, a level editor with workshop support, a random track generator and multiplayer. Even as someone who really doesn't care for racing games, I absolutely adore this game, and it's criminally underrated.

A bunch of my favorites have already been listed, so I'll just mention the one that wasn't: Antichamber, a first person puzzle game that's probably somewhat like Portal in terms of how it requires you to rethink your assumptions about how space works, but it's a very different game, both mechanically and in tone. I don't want to give away too much, but it's a mix of weirdly unsettling elements (although it is by no means a horror game), a design that's actively trolling you in ways that will make you laugh, and mechanics revelations that will have you scream "Wait, I could have done that this whole time?!" It's one of those games that I wish I could delete from my memory and play for the first time again.

-Super Kiwi 64

Retro 3D platformer in the vein of Banjo Kazooie.

Fear and hunger is the most intense RPG-style horror game I have ever played. It's vile. It's fucked up. And it's honestly enthralling. Pay attention to the content warnings on the steam/itch page if you look it up, they're serious. It's like a game in the Berserk universe, but you're NOT Guts.

Gunpoint

S.P.A.Z.

Eufloria

Astroneer

Manifold Garden

Massive Chalice

The Pedestrian

Solar 2

Snakepass

Stacking

I'd personally recommend Rain World, although it has a reputation for being hard, and you'll probably die a lot so this game has to be approached with patience

I don't know what's considered a "hidden gem" so I'm just going to shout out all the indie games that I think are at least 8/10 and recommend:

Rimworld

Dead Cells

Dredge

Inscryption

7 Days to Die

Mount and Blade: Warband

Factorio

Mordhau

FTL: Faster than Light

Raft

Gunfire Reborn

Into the Breach

Hollow Knight

Dome Keeper

DUSK

Golden Light

Vampire Survivors

Void Bastards

The Westport Independent is a censorship simulator, in which you play as the editor of a newspaper, choosing which stories to run and how to edit them in order to avoid angering both the authoritarian government and political radicals who want to take them down, while also appeasing your journalists, who may get angry and quit if you censor their stories too much.

Road 96 was really fun and the soundtrack was great.

Super Auto Pets has replaced Hearthstone for me, it's fantastic.

UNSIGHTED: a great Zelda-like/metroidvania from my home country Brazil, with a bunch of cool ideas about how to traverse the world and dungeons, neat souls-like combat and an anxiety-inducing time limit mechanic. It's set in an apocalyptic world where lesbian robots save the world with the power of love and incredible violence.

ZeroRanger: incredible shmup with awesome soundtrack, delicious anime-vibes, insanely-hard-but-fair gameplay and a surprisingly deep lore leans heavily on buddhist philosophy.

Dandara: another great metroidvania from Brazil. This one has an unique movement mechanic - you don't walk, you almost-instantaneously jump between platforms. The setting is pretty unique, with lots of references to actual brazilian culture and history.

One Way Heroics: an addicting top-down roguelike where you can only walk right, lest you be consumed by the Impending Wall of Doom that's coming from behind. Cooler than it seems.

Hacknet: a text-adventure-kinda-thing where you're a hacker doing hacker things. The interface emulates some old 80's systems and all your commands are actualy Linux commands. Also, the soundtrack slaps.

Hypnospace Outlaw: have you ever wanted to be a moderator forum from the late 90's/early 00's internet? Well, now you can.

20XX: Mega Man meets Rogue.

These are all fast, violent and kind of similar to each other:

• Broforce is a platformer with fully destructible levels where you save the world from Satan's terrorist army. It has more explosions than a Michael Bay movie.
• Hotline Miami and its sequel are tactical top down games about shooting up houses. You die a lot but restarts are automatic so it's no big deal.
• Katana Zero is like Hotline Miami if it was a platformer. You're a cyber ninja who can slow down time and reflect bullets with his katana. The story is interesting and very emotional.
• Ghostrunner is Katana Zero but 3D.

The Long Drive is a cursed game that is surprisingly addicting. Basically it's you, your car, and a 5000km road. That's it. The world is infinitely generated and you don't have to do anything you don't want to do. There's no end game so to speak, just you and the road, if you decide to follow it.

Noita is a very difficult roguelike where "losing is fun" except that it's not always, but many deaths are a learning opportunity, and I find myself starting over even on runs that I live for more than an hour. You have very few healing opportunities, and once your HP gets to 0 it's game over. The game is not fair, but sometimes RNGesus gives you the tools to be a god early on, and even then, it can still be easy to die. Even in full-on god tier runs, there are some things that the player cannot survive. Each playthrough is different, but the map gen is 'basically' the same each game. It's definitely worth your time.

eastward: topdown-zelda like. gameplay wise, though it's quite easy. the story is charming, with all the quirkiness from earthbound, although the main plot is actually more akin to the last of us 1, for some reason, or at least it kinda felt like that to me. the visuals are absolutely gorgeous, as is the soundtrack.

phoenotopia awakening: an action rpg-metroidvania mishmash, and a surprisingly large one at that. if you're into challenging and backtrack heavy games, this one does the job very well.

crosscode: this one's been mentioned a bunch on this thread, and there's also been a post here about it a day or two ago, but i'll still mention it because it's just that good. another topdown-zelda like, but keep in mind that i've seen it described as a puzzle game masquerading as an rpg, which may be a more apt description.

Cassette Beasts! It's a monster collecting game. It adds a lot to the genre (definitely not a clone) and the music is awesome. It got amazing reviews so I'm not sure how hidden it actually is, but I really enjoyed it!

It's admittedly gotten a lot of shout outs with Diablo's release, but Grim Dawn. Just a super great old school ARPG with a fun multi class system. I think I made a mistake by letting it be one of the first in the genre I really got into, so it's kinda tough to find another one that competes.

Not sure if it counts as a hidden gem, but the last indie game that I played through and really enjoyed was Wandersong, a musical platformer where you interact with everything through song. The same dev also put out Chicory, which is like a playable coloring book, but I haven't had a chance to play that one yet.

noita PC - a rougelike witch Sim full of secrets.

Dead Cells everything - Flawless metriodvania rougelite

The Forgotten City PC, PS, XB - A short time loop game that is just fantastic. I think it started as a skyrim mod

Here are some games I have not seen mentioned yet:

Valdis Story: Abyssal City
A 2D metroidvania, with the best combat in the genre. It has infinite combos, parrys, skill cancels, a magic system, and 4 characters that play fundamentally different. You get scores for the bosses, which really motivates me to learn the combat well beyond just finding one thing that works well. If you have any interest in sidescrollers, then this game is for you. Controller is required thou.

Zero Sievert
Ever wish that tarkov had a singleplayer mode? Well look no further, Zero Sievert is a top down extraction shooter, that is still in early access. But whats already on offer is really solid and a lot of fun. It has that tense atmosphere when you find something good and want to extract, without wasting massive amounts of your time.

Ollie Ollie World
A 2d skateboarding game with a unique control scheme, that has a very high skill ceiling. Its score chase, with online leaderboards, daily challanges, seasons, and so on. I like its music and artstyle a lot, but if thats not for you, you can check out the older entries in the series, that look not as 'out there'.

Noita. It's just so good. Easily in my top 5 games of all time.

Don't sure if it's exactly hidden but for me Dungeons III (and 2) has been unexpectedly fun. It takes everything that made Dungeon Keeper and it takes a level higher. Pretty fun game.

Diaries of a Spaceport Janitor is one of my favorite indie games of all time. The city design really captures the feeling of wandering around an unfamiliar, large, bustling place. The diary mechanic at the end of the day is a great way to get in character, and I like that you can decorate the apartment. I did some light data-mining (mostly item info and dialogue strings), and I even have fridge magnets of some of the pixel art!

Depanneur Nocturne is also a great evening’s worth of exploration and vibes, but I mention it because it has a reference to Spaceport Janitor and it made me SO happy when I realized that. :)