Cheat codes:

Enter the following key combinations while playing the game. If the
codes are accepted, you will see a message. Re-enter the code to
disable the cheat effect.

Rifle:
During the game enter the following sequence - Y, A, X, B, Y, UP.

Unlock submachine gun + few other items:
Press Y, A, X, B, Y, Up

Sniper gun + few other items:
Press Y, A, X, B, Y, Right

Rocket launcher + few othes items:
Press Y, A, X, B, Y, Left

Heavy machine gun, miscellaneous items:
Press Y, A, X, B, Y, Down

Shotgun, miscellaneous items:
Press Y, A, X, B, B, Up

Nail gun:
Press Y, A, X, B, A, Left

Unlimited ammo:
Press Y, A, X, B, A, Right

Cycle through spawn points:
Press Y, A, X, B, A, Up

Highest charisma:
Press Y, A, X, B, A, Down

Invisibility:
Press Y, A, X, B, B, Left

Slow motion:
Press Y, A, X, B, B, Right

Fast motion:
Press Y, A, X, B, B, Down

Ragdolls:
Press Y, A, X, B, X, Up

Invincibility:
(PAL game version):
Press Y, A, X, B, X, Right

Enemies can't see you:
Press Y, A, X, B, B, Left

Master Code:
On the pause menu, press A, Y, CIRCLE, Y, A, Y, A, Y, CIRCLE.

Bonus Liberty Island level:
Finish the game once.