Counter-Strike: Global Offensive
105 days 21 hours
8 days 3 hours
The Binding of Isaac
2 days 19 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!
Good luck making money on a free single player game