Ecco the Dolphin

When you are in the "password entry" screen, you can enter these "please" codes to reach the corresponding levels.

Password - Effect

PLEASEHH - City of Forever
PLEASEKK - Deep City
PLEASEOO - Final Fight
PLEASEEE - Ice Zone
PLEASEFF - Island Zone
PLEASEUU - Marble Sea
PLEASEWW - Open Ocean
PLEASEQQ - Origin Beach
PLEASEGG - Pteranadon Pond
PLEASEZZ - Ridge Water
PLEASEII - Undercaves

By entering an even combination of A's and N's in different sequences for your password, you can gain access to a variety of levels within the game. Here's a list of password combinations and the levels they take you to:

AANNNNAA: Deep City
NNNAANAA: The Marble Sea
AAAANNNN: Origin Beach
NNNNAAAA: Island Zone
NNNAAAAN: Undercaves
AAAAAAAA: Undercaves
NNNNNNNN: Welcome to The Machine
AAANNNNA: Welcome to The Machine
ANANAANN: The Vents
AANNAANN: The Vents
NNAANNAA: Dark Water
NANANNAA: Dark Water
ANNANNAA: Dark Water
ANANANAN: Hard Water
NANANANA: Hard Water
AANNANAN: Hard Water
NNAANANA: Hard Water

All other combinations will not work, and will transport Ecco back to the starting screen of the game.

Begin the game and move Ecco left and right. Press Start to pause the game while Ecco is turning. You have to pause it when Ecco is directly facing you. When you do that, press Right, B, C, B, C, Down, C, Up. If you did it right, a cheat menu that has options such as stage select, message test, sound test, and more, will appear.

The Last Fight: KNLMLMLC

Input a valid password, then press Start, and wait for the screen that shows the name of the level you're currently in with your password. Next, hold A and Start. Keep hold of A and Start until Ecco appears on the screen. Now press Start to unpause and you will be invincible.