I've gone all retro:
Heretic
Hexen
Hexen II
You can play both Heretic and Hexen natively as a Universal Binary through the
Doomsday Engine, an open-source frontend for the jHexen/jHeretic/jDoom ports, which allow for all sorts of nifty OpenGL-related coolness.
It's amazing how good Heretic looks with replacement hi-res textures and 3D models; simplistic level geometry and bright color palette aside, you'd mistake it more for an early Quake engine game than the Doom engine-based game it really is.
I also picked up Hexen II after learning about a Universal Binary source port by way of the Hammer of Thyrion project. Very cool.