We all love zombies. Alright, some of us don't love them, but the desire to mow down zombie hordes is nigh universal. With last year's Left 4 Dead, the zombie co-op survival genre was born, and was quickly reinforced by Call of Duty: World at War's Nazi Zombies mode. Killing Floor, from Tripwire Interactive, liberally borrows elements from both of these games and a few others to create a wonderful co-op experience that feels instantly familiar, but becomes unique almost from the moment you take control. With co-op for up to six players being the game's entire focus (its subtitle is "Co-Op Survival Horror", after all), genre (and co-op) fans are in for a real treat.