This one is quite old and most have probably at least heard of it, but it sums up pretty easily! Zombie apocalypse, 4 playable survivors, run'n'gun! Simple fun that's easy to get behind.
Bonus, on Steam it contains all of the stuff from the original Left 4 Dead! 2 games for 1! Aaaaaand Mac compatible! *nudge nudge* Though it is fast paced Tubz so if you want to join in Steam I'm ordering you a mouse. Forcibly.
But! It's on other platforms as well! At least I know it's on the Xbox360. So if you want to play this game with others but have it on a different system speak up! We can't play together if we don't know what everybody has!