At the main menu press L2, R2, L2, R2, Square, Square, Circle, Circle.
All power moves and 120 achievement points:
At the main menu press L1, L1, R2, R2, L2, R2, L1, R2.
At the main menu press Square, Circle, Square, Circle, L2, R2, R2, L2.
At the main menu press Circle, Square, Circle, Square, L1, R1, L2, R2.
Unlimited Electric Batarangs:
At the main menu press L1, R1, L2, R2.
All Weapons and Unlimited Ammunition:
You will start the Plant Electrocution 2 level by sliding down towards some holes in the ground. Fall into the second hole. Glide down to the lit area and pick up the key you find inside. Use the grapnel to exit and finish sliding down. Use the key on the door beside the vines behind the glass. You will find an envelope that has a "C" on it in the room. When you complete the game, you can use this cheat for all weapons and unlimited ammo.
Hint:(Submitted by: CHEATMAsTER)
When you are fighting a bad guy, punch him four times and then block. Keep doing this until the guy is dead. This is useful when fighting bosses like the joker.