Microsoft's Winter 2010 promo for Xbox Live features three games all cooperating together in unique ways, two of the games, feature co-op themselves.
Up first is A World of Keflings, sequel to A Kingdom of Keflings. The avatar based kingdom builder will be released on December 22nd for 800 
. If you decide to purchase it, you'll get access to the demo versions of both Raskulls and ilomilo immediately, the latter of which contains co-op. But that's only the start of the puzzle that's this holiday promo.
Whatever games you do decide to buy will unlock content in the other games. For instance - if you pick up all three - A World of Keflings will have ilo and milo as well as Raskulls characters in the game. ilomilo will have costumes from Raskulls and World of Keflings. Finally Raskulls will have characters from Keflings and ilomilo to play as. Got that? Obviously whatever combination of purchases you choose will have the respective content unlocked in the other titles.
A World of Keflings is a two player local or four player online co-op game where players work together to create their kingdoms and civilizations of Kefling characters using their Xbox Live avatar.
ilomilo is a two player co-op title where players work together to solve block based puzzles and explore the world as ilo and/or milo. It's priced at 800 and is out on January 5th, 2011.
Raskulls is a multiplayer versus party style game - it doesn't contain co-op, but we covered it a bit from PAX East last year. It's also 800 and is out on December 29th.