Unfortunately, at the time there are no codes for Halo!Stay tuned if any are found, but I seriously doubt it. However, there is something you can "unlock," the legendary video, and something for the Campaign.
<More Player Profiles for Campaign Mode>
Press Y to create another player profile for the Campaign(you can beat the game more than three times with more profiles) at the Campaign Screen(or Player Profile screen). This is useful if you have a lot of people playing Halo on the same XboX.
<Alternate ending sequence>
Successfully complete the game in campaign mode on the legendary difficulty setting to view a different and longer ending sequence.
