Left 4 Dead 2 (PC Download) for $6.8, 4-Pack for $20.4

Next generation co-op action gaming from the makers of Half-Life, Portal, Team Fortress and Counter-Strike.
Over 20 new weapons & items headlined by over 10 melee weapons – axe, chainsaw, frying pan, baseball bat – allow you to get up close with the zombies
New survivors. New Story. New dialogue.
Five expansive campaigns for co-operative, Versus and Survival game modes.
An all new multiplayer mode.
Uncommon common infected. Each of the five new campaigns contains at least one new “uncommon common” zombies which are exclusive to that campaign.
AI Director 2.0: Advanced technology dubbed “The AI Director” drove L4D's unique gameplay – customizing enemy population, effects, and music, based upon the players’ performance. L4D 2 features “The AI Director 2.0” which expands the Director’s ability to customize level layout, world objects, weather, and lighting to reflect different times of day.
Stats, rankings, and awards system drives collaborative play