No More Room in Hell the long awaited Source zombie modification.
It's a lot of fun with friends, but it always becomes frustrating because of some things that aren't well made.
The ironsights kinda suck, like they weren't adjusted and the collision detection with melee weapons is also pretty buggy.
But I believe once this mod is out of BETA, this issues will be solved and will be one of the best zombie coop survival games out there.