Here's a nice little surprise if you own Perfect Dark HD on Xbox Live Arcade - a Crackdown 2 agent. How do you get him? All you need to do is play the final version of Crackdown 2 and upon firing up Perfect Dark HD the game will detect the save file and unlock "Agent 4."
Sadly you can't use him through the cooperative campaign as he's only available in the game's Combat Simulator modes. Still, it's nice to have a freebie and some cross game loving from Microsoft's internal studios.