Day-Z is a Zombie mod for Arma 2, and I've been having a blast with it. It's basically a multiplayer survival horror game, except if you die that's it, you have to start all over again from the basic loadout. Whatever loot you had is being worn by whomever killed your sorry ass. Yup, besides the hundreds of zombies trying to kill you, any other player has incentive to kill you for your stuff, because loot is scarce, and therefore valuable. Sometimes people form little bandit gangs and you might be fighting half a dozen trolls, wanting to loot you for your 4 makarov magazines, some water or food(all of which you need to survive), or just to be assholes. Your visibility and sound level all play into whether you are spotted by a zombie or not, and yes, when you shoot one, the others in the area notice, and head that way.
http://forums.somethingawful.com/showthread.php?threadid=3481582
http://dayzmod.com/about.php