In case you guys don't know about it yet, there's this mod for ArmA2 that has pushed ArmA2 to 5th place in Steam's top sellers list. people are buying ArmA2 just to play it (btw, if you do want to try it eventually, DON'T buy it on Steam, it will make things, ugh, unnecessarily complicated, buy a regular version of ArmA2 Operation Arrowhead and combine it with ArmA Free).
the mod is called DayZ, and is arguably "the best zombie survival game of the decade" and "the STALKER MMO that never was" as a lot of people call it. google it, you'll be surprised.
it's in Alpha stage atm (meaning there's almost no content in it), but people play it for the challenge and for the emotions. it's very brutal