Cooperative
Cooperative gameplay (often abbreviated as co-op gaming) is a feature in video games that allows players to work together as teammates against one or more AI opponents. It is distinct from other multiplayer modes, such as competitive multiplayer modes like player versus player or deathmatch. Playing simultaneously allows players to assist one another in many ways: passing weapons or items, healing, providing covering fire in a firefight, and performing cooperative maneuvers such as boosting a teammate up and over obstacles. Read the full article at the Wikipedia.
Team Fortress 2
by Valve
Steam only
Team Fortress 2 (TF2) is the sequel to the game that put class-based, multiplayer team warfare on the map.
Genre: Shooter, Action 
Serious Sam 3: BFE
by Croteam
Steam only
Serious Sam 3: BFE(Before the First Encounter) is a glorious throwback to the golden age of first-person shooters where men were men,...
Genre: Shooter, Action 
Ancient Beast
0.2 (15 Jun 2013)Ancient Beast is a turn based strategy game played online against other people, featuring a wide variety of items and creatures to...
Genre: Card game, Turn-based, Strategy, Role-playing 
Dungeon Defenders
Steam only (20 Dec 2012)Dungeon Defenders is a Tower Defense Action-RPG where you must save the land of Etheria from an Ancient Evil! Create a hero from one...
Genre: Strategy, Role-playing 
Running with Rifles
(2 Apr 2015)RUNNING WITH RIFLES is a top-down sandbox shooter, with emphasis on cooperation, squad leading and being a tiny soldier in a big war.
Genre: Shooter, Action 
Desert Stormfront
1.0 (27 Sep 2012)Desert Stormfront is a Real-Time Strategy (RTS) game situated in the Middle East.
Genre: Strategy 
Shatter
by Sidhe
1.0
(18 Sep 2012)
Shatter is a retro-inspired brick-breaking game that merges familiar action with unique twists and a modern crafted production approach.
Genre: Arcade, Action 
Dustforce
by Hitbox Team
1.0
(18 Sep 2012)
Clean up the world with style! As an acrobatic janitor, you are an adept force against dust and disorder.
Genre: Arcade, Platform 
Trine 2
by Frozenbyte
1.14
(7 Sep 2012)
Trine 2 is a sidescrolling game of action, puzzles and platforming where you play as one of Three Heroes who make their way through...
Genre: Platform, Puzzle, Adventure 
Mari0
by Stabyourself
1.6
(16 Mar 2012)
Mari0 is mix of the jump’n’run and puzzle platformer genres.
Genre: Arcade, Platform, Action 
Zombie Grinder
by Twin Drills
frequently updated (Alpha)
(17 Aug 2013)
Zombie Grinder is a multi-platform cooperative multiplayer arcade game, reminiscent in style and gameplay of retro games like Zombies...
Genre: Arcade, Shoot 'Em Up, Action 
AirMech
by Carbon Games
frequently updated
(4 Dec 2012)
AirMech is a Herzog Zwei-like action-strategy game that puts you in the battle as a giant transforming robot!
From the air, build...
Genre: Shooter, Strategy, Action