Counter-Strike: Global Offensive
7 days 19 hours
2 days 6 hours
6 days 9 hours
Left 4 Dead 2
2 days 20 hours
2 days 9 hours
Can confirm Fistful of Frags is great fun. Adding to the list: the new Unreal Tournament, No More Room In Hell, Halo Online (search for it on reddit, it's basically Halo 3 on PC).
I've never tried: Brawlhalla, Tribes: Ascend, Dirty Bomb or Loadout
Dirty Bomb and Loadout are really cool, you should try them!