Left 4 Dead 2 is the sequel to the best-selling and critically acclaimed cooperative multiplayer smash from Valve, creators of Half-Life, Portal, Team Fortress and Counter-Strike. Introducing melee weapons and new enemies to the originals award-winning gameplay, L4D2 casts players as one of four new Survivors battling zombie hordes as they travel from Savannah, Ga., through New Orleans French Quarter.
Cheaper on Steam ($50 USD + conversion fees). Unless you want physical media.