Windows 8.1 / Windows 8 / Windows 7 / Windows Vista / Windows XP
Until the DayZ Mod came out in early 2012, ARMA II and its expansion, ARMA II: Operation Arrowhead, had essentially gone under the radar. However, with DayZ's preview release earlier in the year, it quickly gained huge traction due to its unique settings and survival horror theme — and for good reason.
The premise with DayZ is that players take on the role of a survivor in the post-Soviet state of Chernarus where a virus has infected everybody — with the exception of other player survivors — and turned them into zombies. While DayZ initially allowed its players to start with a pistol as they worked their way up, players now start the game with nothing but fists, and no hand-to-hand combat with zombies is supported yet, making the game incredibly difficult at first.
To make matters worse, players must eat, drink and maintain a certain blood amount as would be the case in the real world. When attacked, zombies generally break the skin and draw blood, which lowers blood count, and that number keeps dropping as players bleed until they bandage themselves. If the damage is too severe, bandages won't stop the bleeding, meaning they must go to the hospital, which may be overrun by bandits during the nighttime. The less blood the player has, the worse the effect on him.
Overall, the game offers a unique setting that has never been covered by other developers before. Coupled with the added difficulty that requires hunting for weapons and consumables, it makes for something great that has caused millions of players to go out and purchase ARMA II and its expansion for.
Those interested in playing DayZ at the moment must keep in mind that both ARMA II and ARMA II: Operation Arrowhead must be purchased in order to play it. However, the developer also mentioned that a standalone version of the game would be available sometime in 2012.