Various cheats        

L = Leonardo
M = Michaelangelo
D = Donatello
R = Raphael
S = Shredder

Full Health:
     Enter DSRDMRM as a password.

Self Recovery (Regain health slowly):
     Enter DRMSRLR as a password.

Mighty Turtle (Invincibility):
     Enter LSDRRDR as a password.

Squeaking (Alternate hit sounds):
     Enter MLDSRDM as a password.

Pizza Paradise (All items turn into pizzas):
     Enter MRLMRMR as a password.

Toddling (Alternate walk sounds):
     Enter SSSMRDD as a password.

Super-Tough (Enemies have double defense):
     Enter RDSRMRL as a password.

Nightmare (Enemies have double attack):
     Enter SLSDRDL as a password.

Fatal Blow (Enemies have one hit kills):
     Enter LRSRDRD as a password.

Abyss (Lose all health when falling):
     Enter SDSDRLD as a password.

Poison (Lose health slowly):
     Enter DRSLLSR as a password.

Super Offense Power (Double attack):
     Enter SDLSRLL as a password.

Super Defense Power (Double defense):
     Enter LDRMRLM as a password.

Challenge Mode Endurance(No stamina restore):
     Enter MRMDRMD as a password.

Infinite Shuriken:
     Enter RSRLRSM as a password.

No Shuriken:
     Enter RLMRDSL as a password.

New Nexus outfit (Donatello):
     Enter DSLRDRM as a password.

New Nexus outfit (Leonardo):
     Enter LMRMDRD as a password.

New Nexus outfit (Michelangelo):
     Enter MLMRDRM as a password.

New Nexus outfit (Raphael):
     Enter RMSRMDR as a password.