X-Men: The Official Game

Bonus Iceman Danger Room level:
Press Right(2), Left(2), Down, Up, Down, Up, Start at the Cerebro Files menu.
Michael Doyle.

Bonus Nightcrawler Danger Room level:
Press Up(2), Down(2), Left, Right, Left, Right, Start at the Cerebro Files menu.
Fabian Solis and Hal Douglass.

Bonus Wolverine Danger Room level:
Press Down(2), Up(2), Right, Left, Right Left, Start at the Cerebro Files menu.
kmslives.

Danger Room missions:
Progress through the game to unlock the first three Danger Room missions. Six more Danger Room missions are unlocked by collecting all Sentinel files and Weapon X files with all characters.

Comic alternate costume:
Collect all Sentinal Techs with a character to unlock his or her alternate comic costume.

Street alternate costume:
Collect all Weapon X files with a character to unlock his or her alternate street costume.
opifpguns.

Hint: Finding Tech Pieces:
To save time and frustration, run through the level on novice to find all six Tech Pieces, then distribute mutation and save the game. Go back on the higher levels to gain the missing mutations, without worrying about racing around trying to find the pieces (especially on timed missions).
Jason Hicks.

Glitch: Cannot continue if you backtrack:
Continue on to Act 2 without acquiring three Mutation bars in Act 1 for a certain character. Acquire at least two Mutation bars in Act 2 then go back to an Act 1 level and complete it on a higher difficulty for a 100% level completion. You will be given a Mutation bar, but you will not be allowed to add it because you can only add three bars to each character in Act 1. The game will not let you past this screen or regress any of the bars. You will have to restart the game.
Jamie Weir.