Hero mode:
Press L1 + Triangle with a full special meter. 

Invincibility:
Press L1, R1, L1, R1, L1, R1, L1, R1 at the main menu. 
A message at the bottom of the screen will confirm correct code entry.

Unlimited ammunition:
Press L2(4), L1, R1 at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Flame on:
Press Circle, Triangle, Square, Circle, Triangle, Square at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Liquid metal textures:
Press Square, Triangle, Circle, Square, Triangle, Circle at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Big heads:
Press R1(2), L1, R1 at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Small heads:
Press L1(2), R1, L1 at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Pin heads:
Press Square, Circle, Triangle, Square, Circle, Triangle at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Exploding fists:
Press R1(2), Triangle, Square, Circle, R1(2) at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

Hot fists:
Press L1(2), Circle, Square, Triangle, L1(2) at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 

I Got The Power mode:
Press L2, Circle, L2, Circle, L2, Circle at the main menu. 
A message at the bottom of the screen will confirm correct code entry. 
All characters in the game will have blue electrical energy flowing all over their 
bodies. It also seems to give your weapons more knockdown power.