OpenTTD is fantastic. The graphics might give new players pause, but if you like building networks or logistical puzzles, or just like trains, it’s still one of the best in the genre.
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”.
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 still haven’t tried it, but I’ve heard how great Veloren apparently is. It’s an MMO voxel game that takes inspiration from Zelda: Breath of the Wild and is written in Rust.
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
Xonotic (quake-esque FPS, IMO its like quake and halo had a FOSS child), Minetest (a voxel game engine, multiple games are available for it), and Mindustry (sandbox tower defense) are the only ones ive tried so far