So we all know about Arma 2, a military blah blah simulator blah blah helicopter leaves without you and you have to wait 30 minutes before it respawns blahdy blah. It just hit #4 on steam's top selling selling list in the EU and #5 in the US and apparently this is the result of a zombie/survival mod created by one or a few of the developers of ArmA 2 itself. The mod seems to draw inspiration from The Walking Dead tv show featuring new weapons such as a crossbow for silent kills as well as emphasizing more on player to player interactions and atmosphere than it does on the slaughter of zombies. Ammo is scarce and usually only found in the abandoned towns that are scattered throughout Arma's 225 square km map adding some risk vs reward situations where you might find a heap of ammo but then startle a zombie resulting in you having to use it all to fight off the ensuing horde. All your progress is saved on a character server meaning you can stop at any point, switch to a different server and still have all your guns and equipment. If you like games such as the STALKER series but really wanted it to be a COOP/ moderately multiplayer online experience with servers that hold 50+ people or you're German then I encourage you to check it out. http://www.dayzmod.com/
For a proper explanation of what this is read this review from some horror gaming site.