Celebrating its millionth sale, Tripwire Interactive has greatly discounted its post-apocalyptic first-person shooter, Killing Floor. The PC/Mac game is available for $2.99 (85% off) on Steam, while $4.78 gets you the base install along with six downloadable content packs (each download adds four playable characters).

First released as a mod for Unreal Tournament 2004, the original creators were hired by Tripwire to port the add-on to Red Orchestra: Ostfront 41-45. It eventually launched as its own product on Steam in May 2009. Since then, the developer has sold 1.1 million copies of the full game and nearly two million DLC packs.

"This is a major achievement from any game studio as few games, even multi-million-dollar AAA titles, ever sell over one million copies on PC. As a truly independent outfit who have funded ourselves all the way, we are particularly proud of the achievement," the company said, thanking its devoted developers and fans.

Tripwire accompanies its sales figures with some fun in-game stats. As of writing, the company says players have logged a combined 45 million hours on Killing Floor. Some 440 million Zeds (aka zombies) have been killed counting achievements alone, including over 250,000 Christmas Patriarchs (a boss zombie).

"For the next three days we're practically giving the game away with a massive discount of 85% as the Midweek Madness feature on Steam this week. That is the cheapest we’ve ever sold the game for. If you have friends who don't yet play the game, now is the time to educate them on the joys of Killing Floor," Tripwire said.

The gameplay is simple but addictive: you and up to five of your friends dispatch waves of enemies with each round becoming more difficult until you're confronted with a boss specimen (Patriarchs). Scoring kills and surviving rounds earns you in-game cash for additional guns, explosives, ammo, armor and other gear.

Not only is it cheap, but just about any semi-modern system should run the game fine. It's built on a heavily modified version of Unreal Engine 2.5 and its recommended system requirements includes Windows or Mac OS X, a 2.4GHz CPU or better, 1GB of RAM, a DX9 GPU with 128MB of memory and 2GB of storage.