In Desktop Mode there are plenty of free and open source games available, which can be manually added to Steam Launcher.
Shattered Pixel Dungeon is available at a flatpak now, and it’s a really great roguelike.
The open source projects I’ve spent the most time playing are OpenMW and Primehack, but they both require you to have copies of the original games.
Great Game, for now i primarily play it on Android Phone. Easy to begin, but easy to die with one wrong click the game can be lost after hours of trying to get to the amulett!
In case you didn’t know, there’s an official community for it here on Lemmy.
The developer of it is active there and everything!