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 right? I played a lot of Doom in the 90s and it's open source now. Quake too I believe.
Edit: How the hell could I forget Cataclysm: Dark days ahead.
This is a hard list to make, partly because I love open source and there are so few titles I've enjoyed on a regular basis.