Good MP or Co-op games on sale ?

Castle Crashers

Boarder Lands I've only played 1 and 2 but i really enjoyed both

-Don't Starve together
-SpeedRunners
-A.R.K
-L4D2
-PayDay 2
-Resident evil Revelations

XCOM: Enemy Unknown too

Portal 2 and the Saints Row games both have excellent co-op. So does Transformers: War for Cybertron.

Killing Floor

-Battleblock (really recommend it*)
-Don't Starve
-Ibb and Obb (the checkpoints can be REALLY frustrating though)
-Half min hero 2
-Spacefarmers
-Bloody Trapland
-Monaco
-Civ 5

Oh ya and plenty of games that are in FPS are fun too. Like Portal 2 and L4D2. The reason why I didn't list them is because my bestfriend has motion sickness so we can't play them together.

Contagion
Torchlight 2
Orcs Must Die 2
Resident Evil 5
Resident Evil 6
PAYDLC PAYDAY 2
Dragon Ball Xenoverse

Hammerwatch, Risk of Rain if you love the art style. OFC wait for a daily/spec deal :)
Forced is also said to be good, haven't tried it though.

Splinter Cell:Conviction
Trine 1 or 2
Terraria
ArmA 2 Operation Arrowhead (if you like simulators)
Magicka
Cortex Command (is a small game but can be very fun)
Men of War,MoW Assault Squad 1 or 2 (if you like RTS all missions can be played with more than 2 friends)
Half-Life 2 (I think the name of the mod is Synergy that allows it to play the campaign online)

Not sure if any of those are actually on a sale,but it might worth keeping an eye

CSGO, Borderlands 2, Payday 2.

Zombie Kill of the Week

Serious Sam dude, Serious Sam. Be it 1,2 or 3. Doesnt matter. Fun as hell in coop

battlefield bad company 2