Left 4 Dead with 11 other survivors makes for a bloody good time.

I still have about 10 achievements left to get on L4D. One of them being the "Beat one campaign on expert" achievement and another being the "Zombie Genocidiest". Lucky for PC users, there's a way to get the former rather easily. Albeit through a whole lotta hoop-jumping with modding. But in the end, engaging in a 12-person zombie massacre is enough reason to jump through them hoops.

I just wish there were some DLC that would allow this on the Xbox360 version, since that's where I primarily play this