Valve and Tripwire Interactive have announced that Killing Floor is now available for pre-order on Steam. Those who pre-order the title before its May 14 release will receive a 25% discount, knocking the price down to a cool $15.00. In addition to the pre-order announcement, a new trailer has been released showcasing the titles zombie-killing action. The game looks to combine some of our favorite shooter features, including six-player co-op and a persistent perk system. Oh, and zombies. Can't forget the zombies.