Four player co-op is available in all game modes and types including a story mode and an unlockable survival mode. While you can't drop in to a game, you can drop out and not break the session if you aren't the host.
Portal 2 will have a co-op campaign. Players take control of a robot and work through puzzles that require two players to work together to solve.
Team up through the campaign with up to 15 other friends for frantic, old school, shooting action.
The game is playable in 4 player co-op as of the latest update.
Play as the Sorceresses or War Mage through the game's levels in two player co-op.
Sanctum 2 is a sequel 2009's Sanctum, a hybrid FPS/Tower Defense game. To succeed you have to construct and upgrade defensive structures. What makes Sanctum unique from other tower defenses is that when the havoc starts, you jump right into the action with your own weapons and play a key role in the defense.
Group up with three other players online to form a "Wing" and assist each other in exploring sectors of the galaxy, or escorting a friend to their trade destination
Players work together using four different classes of characters to complete quests, gather loot, and trade items. All three systems support online co-op. Local co-op is available on the console versions.
You and some friends can team up against the AI in 2v2, 3v3, or 4v4 matches, either online or split screen.