Cave Story - Excellent indie story-platformer from just before people started being able to make money off those things. Was originally free, then was given an "upgrade" (made worse) by a company that royally screwed over the original dev. The fan community maintains their own set of download links including a modern engine rewrite that works on modern systems. (If you want to actually pay for this, don't, and instead buy Kero Blaster)

Damn, I did not know this. I think I bought that version too. I did buy Kero Blaster though.