Enter the following cheats at the password screen (options/password):
S: Splinter
R: Raphael
M: Michelangelo
L: Leonardo
D: Donatello

GENERAL:
LDRSM - Splinter unlocked for Story mode
LSDRM - Playmates database
SLSRS - Goofy sound effects

RAPHAEL:
SMLSR - Alternate costume
RDSRL - Attacking power
DLDRS - Long bandana, Powerup effects doubled
DSRRS - Shorter bandana, Defensive power
Raphael Special: LMMDR - Very long bandana, Powerup effects doubled

MICHELANGELO:
MMMSL - Alternate costume
MSRMM - Attacking power
LMLMS - Long bandana, Powerup effects doubled
LSSDR - Short bandana, Defensive power
Michelangelo Special: MDMSL - Unlimited fire shuriken. Recovery items are always pizza

DONATELLO:
LMSSL - Alternate costume
DRLDS - Attacking power
SDDLS - Longer bandana, Powerup effects doubled
LRLDD - Shorter bandana, Defensive power

LEONARDO:
MSRLR - Alternate costume
LMLSD - Attacking power
LDDRS - Longer bandana, Powerup effects doubled
SSRSL - Shorter bandana, Defensive power
Leonardo Special: SRRDM - Unlimited shuriken

HINTS TO UNLOCK:

SPLINTER:
Beat Dojo Stage 3 as Leonardo.

CASEY JONES:
Beat Stage 1 as Raphael.

EVIL TURTLEBOT:
Beat Stage 3 as any of the Turtles.

HUN:
Beat Stage 6 as Michelangelo.

CHALLENGE MODE:
Beat Oroku Saki in Story mode.

YOSHI HAMATO:
Beat Challenge Mode as any of the Turtles.

OROKU SAKI:
Beat Oroku Saki as any of the Turtles.

SHREDDER:
Beat Shredder as any of the Turtles.

GEMBU TURTLE POWER:
Beat Story mode with all four Turtles.

HOLIDAY COSTUMES:
If you play on days like Halloween, Christmas Eve or Christmas Day, you'll wear holiday-themed costumes. You can also change your computer clock to this date.