No More Room In Hell, a co-op first-person shooter in the vein of the Left4Dead series or the Call of Duty zombie modes.
Built around the Half Life 2 "Source" graphics engine, this game is available as a free download for PC, Mac and Linux gamers through the Steam client (it does not require ownership of Half Life 2). It is a multi-player only game, meaning there is no single-player component. It features co-op gameplay, dynamic maps and realistic gameplay, meaning one bite might be all that stands between staying alive or joining the undead hordes.
Graphically, No More Room In Hell is not that much of a looker; the models are sort of bland and the levels are bare and poorly lit. Its gameplay can be very unforgiving, which might put off new players. Still, its focus on realistic gameplay - no HUD or crosshairs, scarce ammo and evolving monsters and dynamic levels - should make it very appealing to Zombiepocalypse fans. It is definitely worth a look, especially given its price.