L4D is a game with 4 main characters, fighting their way to leave zombie land, they need to kill manyyyyyyyyyyy zombies, to complete their task... anyways L4D multiplayer system is the best, u can be one of humans( with wepons and healing items ) or u can be zombie with NPC zombies.... Tank( big big zombie) comes every round at least one time and one of players is chosen to be tank... if human people are good they can beat tank......
it's REALISTIC too
but u have have to be carefull