I just want to point out that Tripwire Interactive's co-op zombie shooter Killing Floor is four years old. It came out in May 2009 and the studio is still supporting it with free expansions as well as paid DLC. That's a heck of a level of commitment for any game. Right now you can get Killing Floor for just $3.99/£2.99 which is 80% off the regular price.
The Pier of Pain event runs until July 23rd and features a whole new level and gameplay mode entitled Objective Mode in which players will escort a VIP character whilst fighting off the mutated hordes. In addition, there are two new weapon packs for you to unlock or purchase. Not only that but Killing Floor will be free to try on Steam for a week, starting July 4th. Oh, and all the game's DLC is half price too.
I feel I have to commend Tripwire for its continued support of Killing Floor; I put plenty of hours into the game but haven't played since loosing my regular co-op partners. Seems right now might be the best time to jump back on the wagon.