Various cheats    
     Enter these codes at the map screen.

Cheat code:                       Result:
Left, Right, Left, Right, X       All Driving Upgrades
Up, Down, Up, Down, X             All Fighting Moves
Right, Left, Right, Left, X       All Gunplay Skills
Down, Down, Down, X               Bigger Car
X, Circle, Square, Triangle       Show Nick's Coordinates

Enter these codes as your license plate,
then hold L1 + R1 when you select OK

M1K3 Play as commando
HAWG Play as biker
TFAN Play as gangster
B00B Play as female punk
JASS Play as donkey
TATS Play as tatooed lady
PHAM Play as butcher
MNKY Play as male punk
BRUZ Play as boxer
J1MM Play as corpse
FATT Play as police officer
P1MP Play as pimp
MRFU Play as asian
5WAT Play as S.W.A.T.
B00Z Play as street bum
PIMP Play as pimp
B1G1 Play as your boss
ROSA Play as your partner
HARA Play as asian worker
FUZZ Play as Officer Johnson
HURT_M3 Play as your partner in underwear


Hints (submitted by Ben):

All unarmed upgrades: 
     At  the  map  screen, press Up, Down, Up, Down, X. If you entered
the  code  correctly,  you will hear a sound. Note: Enabling this code
will prevent the training missions from being accessible.

Impound garage cars unlocked: 
     At  the  map  screen,  press Up, Right, Down, Left, Up, X. If you
entered the code correctly, you will hear a sound.

Lesser car mass: 
     While  in  a car, display the map screen, then press Up(3), X. If
you entered the code correctly, you will hear a sound.

Boost: 
     At  the  map  screen,  hold Up and press X(3). If you entered the
code  correctly,  you  will  hear a sound. Greater car mass While in a
car, display the map screen, then press Down(3), X. If you entered the
code correctly, you will hear a sound.

Play as Snoop Dogg: 
     At  the  map screen, press R1, L1, Up, Right, Left, Down, L3, R3,
X,  Square,  Circle,  Triangle. If you entered the code correctly, you
will  hear  a  sound.  Immediately save the game before exiting to the
main menu. Then, go to the mission selection screen and find a mission
with  Snoop's  head  at the top. Enter those missions to play as Snoop
Dogg.

To change back to Nick Kang:
     Select  a  mission  with  Nick's  head  at  the top. Alternately,
collect  all  30  bones during a game to unlock Snoop Dogg. Note: They
are  marked  in  yellow  on  the  map. Play as a bum Enter "B00Z" as a
license  plate.  Note:  Hold  L1  +  R1  when entering the letters and
selecting "OK".

Select car color:
     When  entering a name on a license plate, hold L1 + R1, and press
Left  Analog-stick Up or Left Analog-stick Down to change the color of
the  strip  below  the plate. The color you leave it on when you enter
the name is the color of your car

Bonus FMV sequence:
     Successfully complete the game with all three endings to view the
bonus  FMV sequence. Enter symbols on license plate When entering your
name  on the license plate when starting a new game file, hold L1 + R1
and tap the D-pad Left or Right to input symbols.

Play as blindfolded smoking donkey: 
     Enter "JASS" as a license plate. Note: Hold L1 + R1 when entering
the letters and selecting "OK".

Show hidden dog bones: 
     On  map  Enter  "WHUPASS"  as a license plate. Note: Hold L1 + R1
when entering the letters and selecting "OK".