At the main menu, press L, R, Y, B, L.
At the main menu, press Y, X, Y, X, L, R, L, R.
At the main menu, press X, Y, X, Y, L, R, R, L, L.
All power moves and 120 achievement points:
At the main menu, press L, L, R, R, L, R, L, R.
All Weapons and Infinite Ammunition:
In the beginning of the Plant: Electrocution 2 area you slide down toward a few holes. You want to fall into the second hole. Fall into it and glide down. Glide down there and pick up the item you find inside in the illuminated area. Use the Bat-grapple to exit, jump out of the hole, and finish the slide. The item you picked up should be the key to the door right beside the vines behind the glass. Go inside and fight one of the "Plant Men."You will find an envelope that has a "C" on it in the room. When you finish the game, you can activate this cheat for all weapons and infinite ammunition.
Thanks to Revolution readers iSpy, lance white, and rafael moriel!