Cataclysm DDA is amazing if you are into this stuff.
Free and Open Source Software
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Cataclysm: Dark Days Ahead is pretty astounding, offering an incredible level of detail and degree of freedom, plus just lots of zombies to fight.
Crack Attack! is a loving parody of addictive tetrislike games that is also an extremely addictive tetrislike game.
I really like MineClone2 (a Minetest game).
Definitely Neverball. My kids and I spent so many hours in it.
OpenTTD is good, so is TuxKart, but both have better closed-source alternatives. I don't think Neverball does.
Other than the Valve made stuff, I think my favorite Source engine game is Dark Messiah of Might and Magic. Kicking orcs off cliffs or into spikes is super fun and the basic movement on the engine is pretty much the best FPS has.
Open source != Source engine
Endless Sky and Unciv take up a lot of my time these days. Can't recommend them enough.