The following codes use the d-pad (U=up D=down R=right L=left):

Code - Result:

L,R,D, white, B - Tazer
L,R,D, white, X - Pepper Spray
L,R,D, white, Y - Pistol
L,R,D, black, B - Flamethrower
L,R,D, black, X - Grenade Launcher
L,R,D, black, Y - Mimigun
L,R,D,R trigger, Y - AK-47
L,R,D,R trigger, B - M16
L,R,D,R trigger, X - Grenades
L,R,D,R trigger, A - Molotov cocktails
R,R,R,B - Unlock Freak
R,R,R,Y - Unlock Spanky
R,R,R,A - Unlock Bull
white, L trigger, black, R trigger, A - Invincibility
white, L trigger, black, R trigger, B - Infinite Time
white, L trigger, black, R trigger, X - Decapitating Punches
white, L trigger, black, R trigger, Y - Infinite Ammo
black, R trigger, white, L trigger, Y or A - Big Player
black, R trigger, white, L trigger, B - Back to Normal