Press pause then hold Ctrl and enter the following codes:

Invincibility:
Left, Left, Up, Left, Down, Right, Right, Right
Unlock all Extras and Bonus Levels:
Right, Up, Up, Down, Right, Down, Right
Infinite Lives:
Down, Down, Up, Down, Right, Down, Up, Up

Level Select:
Up, Down, Left, Right, Right, Right, Down, Left
Skip current level:
Right, Left, Up, Up, Up, Right, Left, Left

Turns coins into Mad Garibs:
Right, Left, Right, Left, Up, Up, Left, Right

Unlock all Characters:
Left, Right, Left, Left, Left, Up, Left, Left