Free Co-op games

L4D is around $20, so I find it difficult to think you will find 'free' co-op games just like it. A simple search on the internet would probably solve your problem.

Wait for l4d2, payday 2, killing floor 1 on summer sales. Should be around 5€.

Perhaps you could find something in this search result?

You could try Warframe

Meh. Too much hype in the first one and was very disappointed. I will wait until they launch the game D:

Didn't you guys get Payday: The Heist? I played around 30hs with my friends :) You can also try Dead Island Epidemic, it's not like the other MOBAs.

Marathon games have co-op, but personally I only tried single-player mode.

on Steam?

Alien Swarm

Edit: If you own a Source game, which it appears OP does, there is also

Sven Co-op

and other free mods for Source (which are basically full games) like Synergy

Magicka wizard wars, Team fortress 2

Killing Floor is like L4D's poor brother

I see that u've played 800hrs on dota 2 so i'd recommend trying "Infinite Crisis" :3

Team fortress 2 league of legends Tera Path of exile

No more room in hell

Try Warface

Scorched 3d

Airmech is probably only MOBA style game i like, and it comes with coop vs ai mode

1. Team Fortress 2
2. Hazard Ops
3. Heroes & Generals
4. PlanetSide 2
5. Tom Clancy's Ghost Recon
6. Defiance
7. Firefall
8. Loadout
9. Blacklight: Retribution
10. A.V.A - Alliance of Valiant Arms

I would suggest Payday, but as you think it's not fun.. So, I suggest you play Tom Clancy's franchise, like Rainbow Six Vegas 2 or some Splinter Cell games.

Cry of Fear or try to play MOBA games

http://store.steampowered.com/search/?category2=9#sort_by=Price_ASC&sort_order=ASC&category2=9&category3=9&page=1

All the free/paid coop games on steam!

yep there is
hounds the last hope = some bugs but cool zombie shooter
hazard ops a lot of game mods
http://www.hazardops.com/fr/hq/
http://www.houndsonline.com/en