This game is fucking awesome! It's like being Unicron at a All-You-Can-Eat-Asteroid-Buffet! AWESOME!

If you're looking for zombies, challenging gameplay and the need for teamwork, you gotta try Killing Floor.

With different perks, there will be something for everybody to like.
Furthermore you'll be able to track your progress as you level each perk, and the level s are remembered across servers.

Furthermore, the team behind Killing Floor gives us different seasonel maps and monsters for FREE! Its a must try.

5/5, GREAT VALUE FOR THE DOSH

This game is awesome!
It has a feeling of S.T.A.L.K.E.R. and Silent Hill, where you walk around in a post apocalyptic world, having to scavenge to find ammo, which is both the currency and your weapons ammunition, its a creepy but fun feeling you get, when you have to avoid mutants, demons and traps from other humans.
Most of the game, you'll be using stealth to avoid enemies, blowing out lights, to stay in the shadow.

10/10, you should try it.
Be warned, it has the same requirements as Crysis.