Dancing Jack, PJs Jack costumes:
Beat the game.
Pumpkin King costume:
Beat the Spider boss.
Santa Jack costume:
Beat the Doctor.
Thespian Jack costume:
Get an "A" rank on all levels.
Phantom Jack costume:
Get an "S" rank on all levels and complete both secret chapters.
These cheat codes unlock all kinds of extras for the PS2. You can enter most of them any time you're playing Jack, unless you're in a musical battle. Once you start entering a code, you have three seconds to finish it.
Unlock all the Levels:
L1, L2, L1, L2, L3, R1, R2, R1, R2, R3
Enter this code while "New Game", "Load Game" and "Options" are displayed.
Unlock Phantom Jack, Thespian Jack, Dancing Jack and Pajama Jack:
Up, Down, Left, Right, R3, L3.
Unlock Pumpkin Jack and Santa Jack:
Down, Up, Right, left, L3, R3
Unlimited "Fire" Power for Jack:
Left, Down, Right, Up, Right, Down, Left, Up
Upgrade Soul Robber (Only works when you're above Level 3):
Up, Right, Left, Down, R3, L3
Unlimited Presents for Santa Jack:
Down, Left, Right, Up, L3, R3
R3, L3, R3, Up
Unlock the Statue Collection (Only works in Jack's room):
Up, Up, R3, Up
Play the Musical Battle with One Button (Needs to be re-entered if you save):
Left, Up, Right, Down, Left, R3, L3
Thanks to Revolution readers ed,ZILDRAKIER and hedgedestroyer!