Games like Co Op mode of WARFACE (PVE) (Player vs Environment)

[RAJfromINDIA]

Guys please suggest Free to play ONLINE FPS games on STEAM (or out of steam also) which have CO OP mode in military style (PVE - player vs environment) like WARFACE.
I really like the co op mode of WARFACE where 4 - 5 Players each with different class (medic+sniper+assault+engineer) fight against BOTS in a warzone.

I repeat I only want PVE.
I do not want PVP - (have plenty of options for that already)

Please suggest as much as options as you can...

P.S - (Free to Play, Online fps, PVE - Player vs Environment)

P.P.S - emphasis is on PVE - player vs Environment

[-Killa- Bagmen needed]

Alas that list is meager. Been looking myself, but you have to rly look to find coop. And I have not rly found anything. Black Light Tango down maybe. Sniper Elite V2

[ReBoot]

Team Fortress 2 has a PVE mode which is actually bloody fun! Warframe is not a FIRST person shooter but a PVE-shooter nontheless. Then theres Killimg Floor 2, not free but currently on sale and among the best you can get. Finally, check out Sven Co-op. Rather ancient, but still fun.