Multiplayer fun games?

Hi fellow gamers,

Me and my SO are looking for multi-playing games on steam which are cheap and fun (maybe free). We are having a hard time finding games that both of us find interesting.

Are there any horror/mistery/Fps/MMOrpg multi-playing games on steam that are fun to play with others?

Right now we only play Age of Empires 2 and combat arms and we played Cry of Fear (screen shared) but we would like to expand the collection.

We have played Left 4 dead 2 and it was great but then it got repetitive.

Note : Our computers are not gaming computers (mine specifically) is not very good and I can't handle top quality games

So do you guys have any suggestion?

Thank you in advance.