The floor will kill you!

A fantastic coop game that I started playing is Killing Floor. It is a stand-alone game that is based on a mod of Unreal Tournament 04.

The player is set against waves of increasingly difficult and more numerous enemies culminating in a type of Boss wave. The more players in the game the more difficult the waves. Enemies can start spawning from pretty much anywhere and provides for a very tense environment while playing as you constantly have to watch your back.

Each player earns cash for a kill and this cash can be spent between waves at a shop that appears randomly on the map. That is provided you can reach it before the next wave starts.

There is also a sort of class system, you choose a "perk" at the beginning of the game which gives you specific benefits. You can level up these "perks" while using any of the other ones but you can only receive the benefits of one at a time.

The atmosphere is very tense and can be somewhat claustrophobic with the narrow tunnels and entrances and it is definitely best played with a few friends. There is also an active modding/map-making scene and a very active community. I think this is one well worth the purchase and you will get plenty of hours of enjoyment (or terror) out of it.