DayZ

Found some youtube videos of what may be the greatest game ever...
I have not played it, as far as I know it does not run on Mac.

From the wiki:

Day Z is a zombie apocalypse mod for the military simulator Arma 2 and its standalone expansion pack, ArmA 2: Operation Arrowhead. It assigns the player to the role of human survivor and drops them into a bleak wilderness dotted with villages and small cities.

Throughout the game, the survivors must scavenge for resources such as weapons, ammunition and medical supplies. Unlike other zombie games, food and drink are also needed to satiate hunger and thirst.

Zombies in the game are mostly an annoyance. They will move to the sound of gunfire. The real danger is other players. You can run around murdering people, and become a Bandit. Or you can run around helping people, and become a Hero.

Here are some daring heroics  (NSFW Language)

There are also live streams of the game. As I type they are holding a contest  of two man teams called
Survivor Gamez

Later on I will likely add several you tube channels to my sidebar.