Unlock Costumes
Save a game, go to the save file's main menu and then enter these codes:
[Left], [Right], [Right], [Down], [Right], [Down], [Up], [Left] - Unlock Negative Zone Suit.
[Up], [Right], [Right], [Right], [Left], [Left], [Left], [Down], [Up] - Unlock Iron Spider Suit.
[Right], [Up], [Left], [Right], [Up], [Left], [Right], [Up], [Left], [Right] - Unlock Scarlet Spider Suit.