Cheat codes:
     After  you  have finished the game at any difficult then save the
game and start a new game from the game you just saved. Wait until the
start scene is complete and you should see a control panel on the left
wall.

Press triangle and put in one of the following codes: 

61166 - Max ammo 
10397 - Big heads 
11692 - All weapons 

Art Galleries:
     At  the  main  menu,  press  the following to find an Art Gallery
option in the Options/Extras menu:
Disc 1 - Left, Right, Up, Down, Down, Circle. 
Disc 2 - Up, Up, R1, R1, R1. 
Disc 3 - L1, R2, L1, R2, L1, Square. 
Disc 4 - Circle, Circle, Square, L2, Square 

Here is a list of weapons and the enemies they are most effective on:
Shotgun, assault rifle - Humans
EMP, then shotgun - Fixers
GL's, shotgun - Creatures/monsters
EMP only! - Military Mechs