All Weapons Plus Unarmored Samus (NES Version): Enter JUSTIN BAILEY followed by 12 dashes or 12 spaces at the passcode entry screen. You'll start the game with all weapons plus Samus Aran without her armor. Despite rumors to the contrary, this code was not intentionally added to the game.
Infinite health and missiles: Enter NARPAS SWORD0 000000 000000 at the passcode entry screen.
New game without armor: Enter 000000 000020 000000 000020 at the passcode entry screen.