VCMI - it's re-written from scratch, open source, multiplatform Heroes 3 engine with many improvements and mods manager. It requires some files from the original game though.
it's gotta be Xonotic baby, basically a quake clone that is completely free, a decent sized player based and frag loads of fun!
Got it on me Ubuntu desktop starting out for FOSS games.
I didn't see anyone mention Warzone2100 yet. An excellent RTS, with a neat research system and unit customisation, and fun campaign. They've recently added a couple of new campaigns I haven't played yet, and have enough ongoing dev work on skirmish/multiplayer that some AIs are listed as "X% win rate in AI matches".
QuakeWorld and old school Doom for FPS, Beyond All Reason for RTS, Dungeon Crawl: Stone Soup for roguelikes, Hedgewars for a Worms-like.
0ad, warmux
Super Tux Kart (I play it on Android) is NOT one of them. The physics system is bad at some moments, the items aren't fun to use and some of them ruin the game. The overall game feels amateurish (in a bad way), but one thing that I like is the Windows Car and the drifting. Those are awesome.
I don't know if it counts, but I think the source code of Freespace 2 was released eventually und a non-profit license. The community did some great things with it, especially the Blue Planet campaign.
God, I loved this game.
Armagetron Advanced - a Tron light cycles clone that was a blast for a long time. They even released for free on steam. I've not played in a long time, but now want to jump back on
CDDA
It's definitely a work-in-progress title, but I have really enjoyed SuperTux Advance (AGPL-3.0 according to their github page). It's like SuperTux but with sliding, more playable characters, and more power ups. As of now it's probably my favorite OS game at the moment. -Ace
OpenTyrian
Gauguin - Sudoku-like game for Android (on F-Droid)
The instructions are not clear at first so it's better to start the game a new with lower difficulty.
Teeworlds/DDNet. It has a very simple movement mechanic that is difficult but so satisfying to master, and the skill cap on community maps is crazy
minetest is a rather neat game engine, good bit of gameplay there, even more with the mods and other games you can play.
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