Evil Ryu - Beat the game on the Very Hard difficulty

Ninja of the Future outfit - Complete the game on normal or hard difficulty, then hold L and begin a new game

Plasma Saber Mark II  -  Win the entire game on very hard difficulty.

Plasma Saber  -  Beat the game on Normal to unlock a new costume and plasma saber (Luke, you are now a ninja). To use the new costume, hold down the LEFT TRIGGER and select New Game.

Movie Gallery  -  Beat the game on Normal or Hard to unlock all CG movies.

Skip Cut-Scenes  -  You can skip most of the cut-scenes in Ninja Gaiden by pressing START, A, START.

Unlock all Music in Sound Test - Finish the game once on Normal / Hard

Very Hard Difficulty - Beat the game once on Normal or Hard.

Ninja Gaiden Games:

Ninja Gaiden 1  - Collect 50 scarabs and give them to Maramusa.

Ninja Gaiden 2: The Dark Sword of Chaos - Get Master Ninja on every level in normal mode

Ninja Gaiden 3 - Beat all levels on Very Hard or Hard difficulty while achieving the Master Ninja ranking.
Sent in by Leith & Paul Showers

Beat bosses easier  -  if it is an end of level boss, then use your ART OF INFERNO ninpo, its the best ninpo move and it should be the first you upgrade to level 3. the reason you should use your ninpo is because you beat the boss and finish the level and it gets restored. there is enough jewels of the demon seals to upgrade everything to level 3 here's how you should do it:

JOTDS1:art of inferno
JOTDS2:art of inazuma
JOTDS3:art of inferno
JOTDS4:art of the ice storm
JOTDS5:art of inazuma
JOTDS6:art of the ice storm

Passwords for Old School Ninja Gaiden  -  Enter these at the password screen within the Classic Ninja Gaiden games:

Ninja Gaiden 1:
Act 1- L,X,A,Y,A,X  /  Act 2- A,X,X,R,Y,A  /  Act 3- X,Y,B,R,L,X  /  Act 4- B,R,A,B,Y,Y
Act 5- X,A,Y,X,L,L  /  Act 6- R,Y,R,L,A,X

Ninja Gaiden 2:
Act 1- A,A,B,X,Y,R  /  Act 2- X,L,R,B,B,A  /  Act 3- L,Y,B,A,R,R  Act 4- A,L,X,Y,Y,B
Act 5- R,A,A,B,X,L  /  Act 6- Y,L,R,A,Y,R  /  Act 7- B,B,X,Y,A,L

Ninja Gaiden 3:
Act 1- B,Y,R,L,X,A  /  Act 2- A,R,R,Y,A,L  /  Act 3- L,B,R,Y,L,Y  /  Act 4- R,R,B,A,Y,X
Act 5- Y,A,B,X,R,X  /  Act 6- B,X,X,A,X,Y  /  Act 7- X,R,X,Y,Y,L

Cheat: Skip Cut-Scenes  -  You can skip most of the cut-scenes in Ninja Gaiden by pressing START, A, START.

Unlockable: Movie Gallery  -  Beat the game on Normal or Hard to unlock all CG movies.

Unlockable: Plasma Saber  -  Beat the game on Normal to unlock a new costume and plasma saber (Luke, you are now a ninja). To use the new costume, hold down the LEFT TRIGGER and select New Game.

Unlockable: Plasma Saber Mk II  -  Beat game on Very Hard difficulty.

Unlockable: Very Hard Difficulty  -  Beat the game once on Normal or Hard.

Dual Wielding  -  Ok everyone here is how to get the coolest cheat f ever in ninja gaiden. First beat the game on all difficulties. Then beat normal mode in 2hrs. Then beat hard without saving once and you should have it!!! its amazing lets u use any to weapons u want.

How to get the Dark Dragon Sword  -  The way to get the Dark Dragon Blade is quite simple truthfully, the way to get it is to beat the game and save it. then go back to that same saved file and when you get to chapter 13 or 14 the first time you go see the old man in his shop there will be the sword laying on the floor in there so then you can pick it up and use it. this sword is very powerful and a great sword to have... hope you have fun with this code if you want to call it that.  Also there is an easy way to get money go to a spot in the game and you know there are guys then after you beat them up then go back to the blue roped off area and do it again also its easier when you pull of big hit combos off so my advice to you is to use the numchucks so you can hit more and get a bigger hit combo off.. Have Fun.. :)

Keep the Original Blue Ninja Costume  -  Highlight New Game and Hold down Left Trigger and Right Trigger and Press the A Button you will hear a scream and that's it you will keep the Original Blue Ninja Costume.

Wooden Sword - 5 scarabs
Armlet of Fortune - 10 scarabs
Vigoorian Flail - 15 sacarbs
Armlet of the Moon - 20 scarabs
Dabilahro - 25 scarabs
Armlet of the Sun - 30 scarabs
Armlet of Benediction - 35 scarabs
Jewel of Demon Seal - 40 scarabs
Armlet of Tranquility - 45 scarabs
Dark Dragon Blade - 50 scarabs

Skip to Final Boss on level 16  -  On level 16, get to the boss, and lose to him. Now, continue, and get to the Dark Dragon Blade. During the cutscene when you get it, press start, and the cutscene where you go to the final boss will play.

Breathe Underwater  -  Approach a large body of water and just as you hit it tap A to run on the water. Keep running until you hit a wall and start sinking slowly. Eventually you will hit the bottom and if you keep running you will be able to breathe underwater.