F2P

There's a F2P cowboy FPS-like game on Steam called Fistful of Frags
It's quite fun, you guys should check it out. :)

Also, it uses the Source engine.

Would help if you tells us what you are looking for/what you like.

Blacklight Retribution is fun.

Path of exile is ok if you haven´t played Diablo II, if you had is just like playing Dablo II but missing the cool atmosphere.

Loadout is a good 3rd person FPS although their "funny" style may turn you off.

Dota 2 IF, and only IF you play with friends. If not avoid that hell. (This applies to every MOBA/DOTA style game out there).

Hawken maybe.

dota 2

Loadout

Path of Exile

Defiance in a couple weeks(I am really enjoying it actually...)

HAWKEN

Try HearthStone :)

Star Conflict is best F2P game as for me, try it!!!

The Kingdom of Loathing is a pretty good rpg parody and its style is pretty unique. Alternatively, Vindictus for the great combat. : D

APB Reloaded.