All of these cost nothing so if you like the game consider donating because these are made by people, not corporations.
Yes this was low-effort to type but... they're cool games and deserve attention. Go have a look if they sound interesting.
Mindustry
A sandbox tower defense game
If you like sandbox resource management(ish) games with logic systems and drones with a clean interface, cross-platform support and online multiplayer: this is the game for you. Think along the lines of Factorio.
Veloren
A multiplayer voxel RPG
This game perfectly embodies the spirit of open source.
It's a beautiful game and I love following the updates to see what interesting things the game adds.
Xonotic
The Free and Fast Arena Shooter
Fun.
Minetest
An open source voxel game engine. Play one of our many games, mod a game to your liking, make your own game, or play on a multiplayer server.
Well suited to modders and the creative types.
SuperTuxKart
It's a free wacky 3D, cross-platform multiplayer racing game.
See LibreGameWiki for more.