Carnage Island v3.0
Fast-paced, multilevel map. Ostensibly a circular arena, but most of the action occurs in narrow passageways or the small central room. Best with bigger groups. Update tweaks textures, makes it harder to take a lava bath. (Although there certainly are benefits!) V3 update: The central area is now outdoors. Readme lists other changes, but they don't really seem major in terms of gameplay...
Levels in map "Ala-Map Carnage Island 3.0":
Carnage Island