At the 'New Game' screen, hold L+R and enter the following:

Master Code:
X, Y, B, Up, Right, A, B

Level Select:
Down, Left, Down, Y, Down

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

All Disarms:
B, B, X, X, Right

All Headshot Kills:
B, B, B, Down, Right

Infinite Health:
B, Y, Left, Up, Right

Infinite Shadow Stamina:
X, B, Y, X, Down

Infinite Adrenaline:
Left, Right, Left, X, B

Infinite Armor:
Up, Up, Up, B, Down

Infinite Human Shields:
B, Y, X, Y, B

One Hit Kills:
Y, X, X, X, Left

Unseen by Enemies:
Y, Y, Up, Up, Y

Double Melee Damage:
X, X, Up, Up, B.

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

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

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

Hard Boiled Mode - Enemies more accurate:
Y, B, Left, Left, Circle

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

Boomstick Mode - Infinite Shotgun:
Right, X, X, X, B

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