There's a pair of co-op releases this week, both of which belong to long standing franchises.
Out today is Command and Conquer 4: Tiberian Twilight for the PC. This two player online co-op title puts players in different roles like attack, defense, and support as they work together in the final game of the Tiberian series of games. You can purchase C&C 4 from Amazon for under $50 right here.
Next up, and out tomorrow, is Perfect Dark HD for Xbox Live Arcade. The game features two player online and split screen co-op throughout the whole campaign. Perfect Dark HD costs 800 
and can be downloaded here when available.