FPS Game

I'd recommend Enemy Front. Think of it as an old school cod/moh. Seems it's what you're looking for.

ill just say killing floor ALL THE WAY, and the borderlands games. even if they dont tend to be like those you mentioned they are AWESOME.

if search a shooter game online, try A.V.A. (Advanced Valiant Arms). Is a very good game, very similar to CS. you found easily in the Free-to-play list of Steam.

I'd recommend Rising storm/Red orchestra 2 the only thing is that it has a low player base however I'm Australian and i can get into a full room after school hours.

Wolfenstein: The New Order

Combat Arms (or Combat Arms EU if you aren't in North America), has been my favourite FPS since 2008.

If you like CS than you'll probably like Fistful of Frags. Free, not Free2Play. No Pay2Win. Not even an in-game cosmetics store. Completely 100% Free Western Shooting Action. Normally about 450 people playing. Various maps. 5 gamemodes. FFA, 2 Team, 3 Team, and 4 Team Shootout, and Teamplay. Frequent updates and fixes. FoF is a great free shooter you should really check out if you haven't already.

The Borderlands saga is just AWESOME! It's like "CoD meets Diablo"

Insurgency is really good. There is also battlefield bad company 2.

FEAR combat the free 2 play multiplayer ;)

natural selection 2, arma 3, cod 4

Soldier Front 2 , Rising Storm / Red Orchestra 2 , A.V.A - Alliance of Valiant Arms

TF2, the other games in the CS series, STALKER, the obligatory COD and Battlefields, L4D and L4D2, Red Orchestras 1 and 2, the Far Crys, Unreal and Doom and Quake, Metros