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.
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”.
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
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.
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
It’s a time machine that teleports you to 3am the next day.
Sanmill
Basically nine man’s morris, it’s pretty fun trying to beat progressively harder AI. Each difficulty requires a different (better) strategy. It’s like unlocking levels in a puzzle game.