CrossCode is really good, but it isn’t free. I need to look through my Steam library to see which other games might be open source too. This is the only one I’m aware of, really. ProtonDB might have a list of games that are open source. It’s a great resource for checking compatibility if you’re playing on Linux.
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