DayZ.
If you're unfamiliar with it, it's a mod for Arma 2 that has skyrocketed in popularity. Arma 2, if you are unfamiliar with it, is a military simulator. DayZ is the first zombie game that can also be classed as a simulator. The map is about 250 square kilometers, populated by up to 64 players divided into two factions, (military types and rebels) and anywhere from 300-400 zombies.
It's the shit. It's got a learning curve from hell, and you need a nice rig for the game to look convincing and run well, but man is it good fun. You start with a bandage, a bottle of water, and a flashlight, and your first immediate concern is finding a firearm. Small farm sheds or houses are a good first bet, not as densely populated with zombies as towns are, and your odds of finding a gun and ammo without being mauled to death are a bit better. You have three resources to maintain- food, water, and blood. The goal is to survive the first day. Good friggin luck, you'll need it.