Classic NES Series: Metroid Cheats for Game Boy Advance

Refill energy easily:

Go to a pipe that has enemies constantly appearing at ground level with no other enemies around. Just keep shooting and the enemies will fly into your bullets, leaving energy and other items.
Fight Kraid:

Use the Ice Beam to freeze the center spike in mid-air as he shoots. Then, use your missiles to shoot the bare spot where the spike was located.
Super password:

Enter NARPAS SWORD0 0000_ _ _ _ _ _ _ _ as a password. Note: "_" indicates a space, and except for the letter "O" in "SWORD", the remaining characters are the number zero. You will start at the beginning of the game with unlimited life, unlimited missiles, and all upgrades. The word in this is "NAR PASSWORD". "NAR" is an abbreviation for the name of the person who designed the password system, Tohru Narihiro.
Alternate game:

Enter 999999 999999 KKKKKK KKKKKK as a password with no Varia suit and a different ending.
Quick save:

Press Start to pause game play, then press Select + Up to display your current password.
Play as Samus without her suit:

Complete the first quest without dying and with all the items (ice beam as your gun weapon). Press Start after the credits have completed. If you have completed the game correctly, Samus will take off her suit in the ending. You will start with all your items you gained in the previous quest minus your missiles and suit in the next game.