Would you like to play a game... together? Sure. How about Global Thurmonuclear War?

The latest and greatest in this classic strategy game series allows players to cooperate or compete across the entirety of human history.

Valve's cooperative zombie shooter's unique twist allows for asymmetric play where survivors pit themselves against someone controlling special zombies.

An old school style 2D roguelike dungeon crawler based around 4-player coop. Requires (and includes) Mono to run on OSX but supports crossplatform play.