Left 4 Dead 2
If shooting zombies is your thing, Left 4 Dead 2 is your game. Team up with three friends to play through any of the five-part campaign maps, including those from the first game, or go head to head with your friends picking sides as zombies or survivors. The addition of L4D2's melee weapons into the original game's maps is a nice touch as well. If you're looking for a stiffer challenge, see how long your group can hold off the zombie hordes in survival mode.