If the sixteen co-op titles coming out this month aren't doing it for you, why not check out a few older games on the cheap? The new Xbox Live Arcade Deal of the Week has five better-than-good co-op offerings for 50 to 60% off their regular price. Lara Croft and the Guardian of Light, Bionic Commando: Rearmed, Bionic Commando: Rearmed 2, Geometry Wars: Retro Evolved 2, and Warhammer 40k: Kill Team are all on sale. Follow each game's link for co-op details.
You can see the full list of games after the break.
These links will take you to the XBL Marketplace. Not all games listed below support co-op. (In fact, only the games I mentioned above have cooperative play.) I included a dollar amount for those of you who don't like to deal with math. These prices should be good for a week.
Gotham City Impostors, 600 MP (50% off) $7.50
I Am Alive, 480 MP (60% off) $6.00
Lara Croft The Guardian of Light, 600 MP (50% off) $7.50
Geometry Wars: Retro Evolved 2, 200 MP (75% off) $2.50
Warhammer 40,000: Kill Team, 320 MP (60% off) $4.00
HELL YEAH! Wrath of the Dead Rabbit, 480 MP (60% off) $6.00
Bionic Commando Rearmed, 320 MP (60% off) $4.00
Bionic Commando Rearmed 2, 480 MP (60% off) $6.00
Black Knight Sword, 400 MP (50% off) $5.00
Source: Marketplace.xbox.com