DayZ is a mod for the game ArmA 2. Never heard of either before today, but after checking out some vids of it, I really want to get it.
You spawn randomly on this huge map with very little resources and then do whatever you want. Head into town and gather supplies, roam the countryside, team up with other survivors or gun them down and steal their stuff. If you die, it's permadeath and you have to make a new character from scratch.
This video series gives a pretty good idea of what the experience is like when you team up with others and it looks hella fun.
Mod is still in alpha, so it's very rough and the zombies all share a stock animation and scream which kinda kills the immersion, but the feeling of being completely open and exposed and vulnerable looks awesome.
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Still the number one reason for the success of the Internet.
http://img.photobucket.com/albums/v192/FAH_Frenchie/YANsig02.jpg