Thinking back I can probably list all the open source games I've enjoyed on one hand. Shattered Pixel Dungeon is the one I enjoy daily, also enjoyed the predecessor. Play it every time I'm on the toilet. ;) Konquest was a really fun old game that came with KDE back in the late 90s, early 2000s. I assume it was open source, I was just a kid playing it. Nethack of course, enjoyed many hours in that dungeon. Doom counts…
I played HEAPS of Konquest back in the day.