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.
Capsized
1.0 (28 May 2013)Capsized is a fast paced 2D platformer focused on intense action and exploration.
Genre: Platform, Action 
Rise of the Ravager
Rise of the Ravager is arcade action best described as a blending of Guitar Hero and Missile Command.
Genre: Arcade, Shooter 
Left 4 Dead 2
by Valve
Steam
(4 Jul 2013)
Set in the zombie apocalypse, Left 4 Dead 2 (L4D2) is the highly anticipated sequel to the award-winning Left 4 Dead, the #1 co-op game of 2008.
Genre: Shooter, Action 
Wargame: European Escalation
Release (27 Feb 2013)Wargame - European Escalation is a real-time strategy game on PC, that maintains the two core values of the classic wargame (depth...
Genre: Strategy 
Guns of Icarus Online
by Muse Games
Guns of Icarus Online is a team-based multiplayer online airship combat game set in a steampunk/dieselpunk-inspired, post-apocalyptic...
Genre: Shooter, Action 
Killing Floor
Linux release (16 Mar 2013)Killing Floor is a Co-op Survival Horror FPS set in the devastated cities and countryside of England after a series of cloning experiments...
Genre: Shooter, Action 
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