Do you want to get this mod? Well first, you need Arma II: Combined Operations. After you get that, download the DayZ mod (which is free) from here. Run through the installer and you can now successfully play this game.
Not convinced enough to buy it? Well picture yourself building a car from parts you scavenged all across the enormous map (which is 225 km2). You can actually drive it after wards. If you travel a lot, you can find downed helicopters and repair them. That's right, you can fly helicopters and kill anyone in sight.