What is some online co-op game that I can play with my friends?

Nothing comes with the graphics; just want a good PC co-game to play with my friends.

We’ve played Minecraft


Garry’s Mod


Everquest(although I don’t really like RPGs)

Killing Floor

Not a big fan of MMOs also

And that’s all I can think of now. I’m sure there’s much more but I forgot.

So any suggestions, folks?