I would have to agree with KittyKat, League is very popular and it is all that some of my friends talk about. It isn't an FPS, but it is a free online MMO.
I've heard this is good as well, never played it but from what I've seen its better than Call of Duty and it's free. Then again, 90% of FPS games are better than Call of Duty.
There is a new game that just came out on steam called "Loudout" Its pretty good , Its just like TF2 so if you like TF2 then i guess you will like Loudout