Recommend some fun multiplayer games

Payday 2 and Left for Dead 2

Robot Roller-Derby Disco Dodgeball. More long term if you're playing with a consistent group of friends I'd recommend Dungeon Defenders.

100% Orange Juice!

No Valve or f2p?
Hmm, Killing Floor?

Rust.....It's preferable to play it with friends though as this game is full of crazy Russians (no racism intended) who'd jump on you the moment your character re-spawn or stealing your house & resources which take much time & luck to be gathered.

Unreal Tournament

Magica, especially when the second one is coming out!

Speedrunners. You can try offline for free, but of course, so much better online.

Bad Rats Online Edition is pretty good