Years before Halo’s Firefight, Gears of War 2’s Horde mode, and Call of Duty’s Zombie mode made this style of play popular, Killing Floor made co-op survival popular ammongst FPS gamers. Originally conceived as a total conversion mod for Unreal Tournament 2004, The Killing Floor spun off into its own retail game. Playing as one of a group of soldiers, you stave off an invasion of mutated people with a variety of weapons. You must survive each round by any means necessary, be it making it to high ground, barracading yourself in, or simply shooting everything that moves. In between rounds you can buy weapons from the merchant, and you simply play to outdo your score. If you enjoy Left 4 Dead’s brand of co-op survival horror, you’ll love Killing Floor. It’s free to play on Steam this weekend, and if you like what you play, you can purchase it for just $5.