Enter the following at the password screen:

Password - Effect

MXND123RTH - All doors unlocked
SHJY770DEC - Developer's Picture
JDGF035JUO - Extra Life
JLKF0405KDA - Level 2
BGGD153GHS - Level 3
NDGJ7654KTG - Level 4
NKGL493FJS - Undetectable