The following great single-player features can be unlocked by pausing the game during gameplay and holding R1 and entering the corresponding codes.
Unlock 3rd Person View:
Press Square, Circle, Square, Circle, L3, L3, Triangle, X, Triangle, X, R3, R3.
Unlock Super Ragdoll Mode:
Press X, X, Circle, Circle, Square, Square, Triangle, Triangle, X, Circle, Square, Triangle.
Unlock Big Head Mode:
Press O, Square, X, Triangle, L3, Triangle, X, Square, O, R3.
Unlock One Hit Kills Mode:
Press L3, R3, L3, R3, X, O, L3, R3, L3, R3 Square, Triangle.