Godzilla Cheats (Nes)

At the password select screen, put in NO characters. Go directly to "END". This will take
you to the Earth level, with no monsters or any characters whatsoever visible. Press
SELECT to bring up the password screen. Since you have no allies left, pressing SELECT to
undo the password screen will automatically result in a GAME OVER.
  Not useful, but weird and sort of fun.

To fight all final monsters, enter "DESTR0Y ALL M0NSTERS."
  To meet the Monster Zero at the start of the game, enter "M0NSTER 0."
  To view the opening and closing scenes, enter "START T0 END."
  For the sound test, enter the password "S0UND"

  Just enter the letters to your password and ignore any irregular spacing that occurs
between the words. 
  Notes: Use the number 0, not the letter O. There are 4 lines to enter these passwords
on. Always start entering the password on the first line, and leave any unused spaces
blank.