"arena_excavation.bsp" or "The Frost Excavation"
(I'll be uploading it to my own webspace later)
The level is set in a small section of an office-ish sort of building. It's supposed to be buried underground, so there're walls of dirt blocking off the exits and the areas outside the windows. To make the dirt more... dirt-like, I used the displacement tool, so it looks somewhat better than just a flat block with a dirt texture slapped on it.
There are spawn points, starting weapons, and AI nodes for the enemies, so it plays okay. The fact that the grunts have no hesitation about using grenades in such close quarters is rather vexing, but it's more than tolerable. Note that there is a "secret" weapons cache that can be found by smashing the wall across from the two soda machines a couple times. You can tell which section of wall it is because the texture is slightly misaligned.
One final note: Almost everything in the map save for the walls, dirt, and windows is either damageable or a physics object, EXCEPT for much of the contents of the cafeteria-esque room.
EDIT: The link above now points to a version with better AI nodes. Improved layout and lighting will come later today.
FURTHER EDIT-OSITY: I've completely redone the lighting, added large holes in the walls that provide much better connectivity (once you smash away the wooden bits with one melee hit), removed some furniture for easier movement, and redone the AI. I think it's actually a decent map now, but I'll leave that judgment to you.