This is a zombie apocalypse mod that has gained a lot of attention and praise lately. It's developed by one guy, the production values are quite trashy, the mechanics are janky, yet the concept and execution is so compelling it makes people easily look past these shortcomings.
You start off in a fairly large sandbox map with no goal other than to survive by scavenging for food and water while defending yourself from the undead, or other players who act hostile towards you. Much like Minecraft, the game becomes pretty much what you decide to make of it. There are several servers usually supporting up to 50 players each, and you can switch between them as you please, but once you die your character and all your stuff is gone forever and you have to start over from scratch.
The mod has become so popular that the servers are constantly filled to the breaking point, and there's already talk about making it a fully fledged standalone game. The mod has caused Steam sales of ArmA 2 to skyrocket into the top 10.
I recommend checking out the youtube Let's Play series Days Ahead to get a good feel of what the game is all about. It's really long, but so interesting to just sit and watch it almost becomes like a movie.
http://www.youtube.com/watch?v=KRpPIyJDSEQ