Killing Floor is a co-op zombie survival style FPS. You play as a special forces member sent in to take out the specimens created by the patriarch. You have a number of classes to choose from all with their own benifits. There is also a decent amount of weapons at your disposal (Lever Rifle, dual deagles :D) as you fight your way with your team mates through the wave of deranged experiments, meeting up with the trader at the end to buy more weapons, ammo and armour.
At the moment steam is running the Killing Floor free weekend, so if you have fast enough internet download it and play the last day out! Its a really fun game and you feel so cool when you headshot a monster from 300 metres away and it goes into slomo.
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I am definetly buying this game after playing it!