Left 4 Dead 2 is a Co-Op first person shooter from Valve. In it, 4 Survivors try to make it through multiple campaigns with different game modes by using a variety of weapons.
Borderlands 2 is the sequel to Borderlands, a popular FPS/RPG. It takes place on a Sci-fi wasteland planet and features various classes and a extremely extensive loot system with "literally millions of weapons."
Team Fortress 2: Mann vs. Machine is a free-to-play multi-platform team-based first-person shooter co-op mode included in the Team Fortress 2 video game. Being cooperation based breaks from the core gameplay of the game, which tends to be team based PvP.