Left 4 Dead is a great game! It's a Team Based Zombie Survival First Person Shooter. Or as I like to call it: TBZSFPS. You and your friends play as one of 4 zombie apocalypse survivors. You have Bill, the grizzled war vet, chain smoking curmudgeon who is like a crazy uncle. You have Louis, the disgruntled corporate drone who is finally taking charge of his life. You have Zoey, a pretty girl with grit, a terrible attitude, and terrific aim. And then you have Francis.
Left4Dead is a blast and the best TBZSFPS ever. Of course it's the only TBZSFPS ever but I digress. It's appropriately rated M for mature. You can find it on Steam and when I and my friends are not playing WoW, we're blasting zombies.