Enter one of the following passwords in the original Metroid.

Samus without suit at start
Enter 000000 000020 000000 000020 as a password.

Samus without suit at Norfair
Enter SAMUS8 RIDLEY 444444 444444 as a password. You will start at Norfair with Suit-less Samus, infinite Missiles, blonde and brown hair color, and no items or upgrades except for missiles.

Samus without suit at Kraid's Lair
Enter JUSTIN BAILEY ------ ------ as a password. You will start at Kraid's Lair with the Long Beam, Ice Beam, Bombs, High Jump Boots, Screw Attack, Varia, Suit-less Samus, and 255 Missiles.

Invincibility
Enter NARPAS SWORD0 000000 000000 as a password. This allows you to start with everything and invincibility. The Bosses are not defeated, and you can play the game normally, but invincible.

Best ending, no suit, at end
Enter X----- --N?WO dV-Gm9 W01GMI as a password.

Debug mode
Enter BOMBS_ MISSLE SCREW_ ATACK_ as a password:

Hard mode
Enter 999999 999999 KKKKKK KKKKKK as a password.

Morph Ball and Bombs at start
Enter 000000 000000 4G0000 00000H as a password.

Mother Brain gone
Enter SAMUS_ BEATS_/MOTHER BRAIN_ as a password. You will start in Tourian, with Mother Brain gone.