Pause Your Game hold L,R and press Y,X,X,X to unlock Frodo.

On all these cheats you MUST pause and be holding the L and R buttons the whole time.

Aragorn:
Up, B, Y, A, Give Aragorn 1,000 Experience Points
Y, B, B, Y, Replenish Aragorn's Arrows
Up, B, Y, Down, Aqcuire four upgrades immediately, when leveled up high enough. Upgrades: Dark Deliverance, Shield Cleaver, Lightning Strike, and Swift Justice.

Legolas's Gimli's and Sam's cheats can be used more then once.

Legolas, Gimli, and Sam:
A, Y, Up, A, Give Legolas 1,000 experience.
X, X, Y, A, Give Gimli 1,000 EP
T, A, Down, A, Give Sam 1,000 EP

Gandalf:
X, Y, Up, Down, Give Gandalf 1,000 EP
Y, Up, B, A, Bring Gandalf to level 4

Unlock Frodo:
Y, X, X, X, Unlocks Frodo for use.
Y, Up, Down, X, Brings Frodo up to level 2

Secret Characters:
Frodo
Merry
Pippin
Faramir

You have to have beaten The Cracks of Doom to get, Merry, Pippin, and Faramir.

First you must beat the game then pause during a level and hold down L&R

Restore Health B,B,X,X
Perfect Mode X,down,Y,A
All Upgrades up,down,Y,B
Infinite Missile B,B,down,X
Invulnerable B,X,B,up