Im playing DayZ Standalone. Awesome.
Same, built my first gaming PC a while ago and I'm absolutely hooked on this.
Picked up Arma 2 in the steam sales so I'm gonna give the mod a try at some point.
Other than that I'm playing BF3 (it was free) and re-playing some of my xbox 360 games like skyrim and far cry 3.
Oh and black mesa