Press Esc and type "glazed donut". Now, you can enter the following codes:
F3: previous level
F4: skip level
Alt + I: invisio power
Alt + K: beach ball
Alt + T: morph
Alt + F12: add 5 lives in single player mode
Alt + 8: infinite bombs in single player mode
Alt + B: cloak
Alt + S: enable scrolling
Alt + F7: change appearance