The free version is on steam but thats just the original ArmA not operation arrowhead, can also get it from other places on net as well i imagine.
It's on steam but can probably get it cheaper elsewhere. It's ArmA 2: Operation Arrowhead
There's 2 expansions as well, British Armed Forces, and Private Military Company.
I like it because it's more realistic, especially when you've got tanks and air support all human controlled online as well as ya foot troops