While at the start of the Plant Electrocution 2 Level, you will see some holes in the ground.  Fall and glide into the second hole, pick up the item in the lit up area.  Grapple out of the hole and finish going down the slide.  The item will unlock the door to the right next to the vines behind the glass.  Go into the door and beat Plant Man.  You will then get an envelope labeled 'C'.  This will activate the code when you beat the game.

Unlock Unlimited Batarangs

While you are at the Main Menu, press L, R, Y, X.

Unlock All FMV Sequences

While you are at the Main Menu, press L, L, X, X, Y, X, L, R.

Unlock Everything

While you are at the Main Menu, press L, R, L, R, X, X, Y, Y.

Unlock All Power Moves/ 120 Achievement Points

While you are at the Main Menu, press L, L, R, R, L, R, L, R.

Unlock Unlimited Handcuffs

While you are at the Main Menu, press X, Y, X, Y, L, R, R, L, L.