Death Jr.

PSP

Pause the game, hold L + R, and enter the following codes:

Up, Up, Down, Down, Left, Left, Right, Right, Square, Triangle	Invincibility
Triangle, Triangle, X, X, Square, Circle, Square, Circle, Right, Down	Unlimited Ammo
Up, Up, Down, Down, Left, Right, Left, Right, X, O	All Weapons and Weapon Upgrades
Triangle, Triangle, X, X, Square, Circle, Square, Circle, Down, Right	Refill Ammo

Thanks to Revolution reader Sandoval!