Altered Beast Codes:
Hold A, press Start (on title screen) - Cheat Menu

Hold X + A + B + Down + Left, press Start (on title screen) - Choose your Altered Beast

Hold X, press Start (on title screen) - Continue on Level After Dying

Hold A, press Start; pick level, then hold X + Start (on title screen) - Level Select

Hold X + B + Up/Right, press Start (on title screen) - Sound Test

Golden Axe Codes:
Hold Down/Left, press A + Start (on Select Player screen) - Level Select

Hold Down/Left, press X + B (on Select Player screen) - Nine Credits

Vectorman Codes:
X, A, X, B, X, A, A (when paused) - Boundaries

X, A, A, X, Down, X, A, A, X (on Options screen) - Cheat Menu

B, X, Left, Left, X, B, X, A (when paused) - Go Anywhere as Cursor

X, A, X, A, X, Down, X, A, Right, X (when paused) - Life Refill

A, X, Left, Left or A, X, Left, Down (when paused) - Map Coordinates

Down, Right, X, B, Up, Left, X (when paused) - Slow Motion

Streets of Rage 2 Codes:
Hold X + A on Controller B while selecting Options (on menu main) - Extra Lives and Level Select

Streets of Rage: Hold Down/Right + A on pad 1, left + X on pad 2, then B on pad 2 Two players as the same character

Sonic The Hedgehog Codes:
Up, Down, Left, Right, hold A, press Start (on title screen) - Level Select

Virtua Cop 2 Codes:
X, A, Y, B, B, B, Up (5x) (on controller D, after completing game on Easy) - Big Head Mode

L, Left, L, Right, L, Down, L, Up, R, Left, R, Right, R, Down, R, Up (on controller D, after completing game on Normal) - Mirror Mode

B, Left, X, Y, X, Y, B, A, B, A, Right, Left, Right, Left (on controller D, after completing game on Hard) - Random Mode

B (when paused) - Access All Guns

Hold Start (on file screen) - Play as Janet

Phantasy Star 2: Slow Motion:
While in the middle of a game, pause the game. Hold the A button. While holding A, now Move around using the D-Pad. You will be moving in slow motion.

Revenge Of Shinobi: Infinite Shurikens:
Go to the Options Screen and move down to the Shurikens option. Set the number to 00. Now wait. After waiting you will see the number 00 turn into an Infinite symbol. You will now have Infinite Shurikens throughout the whole game.

Sonic The Hedgehog: Debug Mode:
At the title screen, press Up, A, Down, A, Left, A, Right, A, Start, then hold X. Continue to hold X until the game begins. Screen coordinates will appear instead of a score and the number of objects displayed will appear instead of the time remaining. Whilst in debug mode you can do these things by pressing the right controller buttons...

Change into Object - Press A
Change Object - Press X
Place object - Press B
Change back to Sonic - Press A again
Invincibility - Sonic is always invincible in debug mode
Restart - Pause the game and press X.
Frame Advance - Pause the game and press B.
Slow-motion - Pause the game and hold A.

Revenge of Shinobi: End Bonus:
Complete any level with a number of shurikens equal to 11 times the number of remaining lives to add 30,000 bonus points to your score.

Phantasy Star 2: No Opponents in Dungeons:
Continuously press B to display the control menu and press A to close it while holding the D-pad in any direction.

Sega Swirl: Bigger Score:
Execute a big move, then pause the game during the animation and choose to restart. The move you just did will register on your new game.