Ninja Gaiden

Cheat mode:
Hold Down on controller two + hold Up on controller one, then hold 1 + 2 on controllers one and two at the main menu, while "Press Start" is blinking.

Unlimited force:
Enter MFREE

Invincibility:
Enter NOCIDE

View credits:
Enter MONTY

View ending:
Enter AKUSX

Level passwords:

Level	Password
1	LEGND
2	NINJA
3	GIDEN
4	DRGON
5	SWORD

출처 : http://www.gamewinners.com/SMS/NinjaGaiden.htm