At the 'New Game' screen, hold L1+L2+R1+R2 and enter the following:

Level Select:
Down, Left, Down, , Down

All Guns, Infinite Ammo:
Up, Left, Down, Right,

All Enemies Disarmed:
Right, , Left, , .

All Disarms:
, , , , Right

All Headshot Kills:
, , , Down, Right

Infinite Health:
, , Left, Up, Right

Infinite Shadow Stamina:
, , , , Down

Infinite Adrenaline:
Left, Right, Left, ,

Infinite Armor:
Up, Up, Up, , Down

Infinite Human Shields:
, , , ,

One Hit Kills:
, , , , Left

Unseen by Enemies:
, , Up, Up,

Double Melee Damage:
, , Up, Up, .

Stronger Punches and Kicks:
Down, Cicle, Left, Left, Left

Chow Yun Jack Mode - Dual guns:
, , Up, Up, Up

Wussy Mode - Enemies less accurate:
, Left, , Up, Down.

Hard Boiled Mode - Enemies more accurate:
, , Left, Left,

Precursor Mode - Turns off targeting:
Up, Up, Down, Down, Up.

Boomstick Mode - Infinite Shotgun:
Right, , , ,

Thanks to Revolution readers Prince B., MÏKÈ, Ray Righteous, sean, Shmit, Jonathan Montufar, Ares, dude gibson, R3D, Brandon, ALBERTO, Tha G, and Rob!