Enter the following at the password screen:

S: Splinter
R: Raphael
M: Michaelangelo
L: Leonardo
D:Donatello

SRSMM	Donatello alternate costume
DRLDS	Donatello more attack Power
SMLLR	Donatello's power-up effects are doubled
LLSMR	Donatello's defense doubles
RMMLL	Leonardo alternate costume
LMLSD	Leonardo more attack power
MSSRD	Leonardo unlimited shuriken
SLDDS	Leonardo's power-up effects are doubled
MLLSR	Leonardo's defense doubles
DLDSM	Michelangelo alternate costume
MSRMM	Michelangelo more attack power
MRRSL	Michelangelo unlimited fire shuriken/Items are always pizza
SMRDL	Michelangelo's power-up effects are doubled
MRLLD	Michelangelo's defense doubles
RDSRL	Raphael more attack power
LRLMS	Raphael’s power-up effects are doubled
LMSDD	Raphael alternate costume
LSMDR	Raphael's defense doubles
LSRDM	Playmates toy Database
LLMSR	Unlock Splinter

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 internal system clock to get it to work.

Thanks to Revolution readers Chris Pelleiter and TalonPeacecraft!