Fungames to play with my friends? *Online*

BattleBlock Theater, Castle Crashers and mount your friend.

• Chivalry: Medieval Warfare
• Risk of Rain
• Lethal League
• Rocket League

Listed games with 4-packs, so you can save a bit of money :P

What about Spelunky? You can local co-op with 3 friends and get into the roguelike madness :)

Mountain

Rocket League

Killing floor 2!

Wyv and Keep's got a 2-player co-op which is quite good when you want something which is more puzzly/slower paced, but still with a good sense of fun.

Happy Wars is fun with a team of friends, but I have no idea how alive the game is now - matchmaking at the higher levels seems pretty dead, but maybe the lower level games still fill fast.

Planetside 2 takes a little while to get into, but playing in a group is pretty fun, especially when you all get in a vehicle and rack up kills (until your mate ends up running into a pile of mines and blowing you all up.) it's also free!

Awesomenauts, Bloodsports.TV, Borderlands 1 +2, Dead Island, E.Y.E Divine Cybermancy, How To Survive, Lost Planet 2, Magicka, Mount & Blade:Warband, ORION:Prelude, Payday:The Heist, Resident Evil 5, Torchlight II, Trine 1 +2.

Thanks for all the suggestions guys! :)