Easy Special Moves:
While fighting in a single player battle, press Up, Up, Down, Down, Left, Right, Left, Right, B, A. If you entered the code in the right way then you will hear a sound. This allows you to pull off special moves with easy movements. For Instance, wen playing as Ryu press Forward + Punch to send out a fireball. This will make the game easier to play with the small arrow pad. Press Select to toggle "easy special mode" on or off.

Character Gallery:
Finish the game in the highest skill level (8) without using a continue to unlock a gallery mode where you can veiw all the game art. To access the gallery press the (R) button on the main menu. You may be able to access the gallery mode even if you have used contiunes, but i don't normally need them and I finished the game on skill 8 with out continues first time I tried it so I can't be sure.

Play as Akuma/Gouki:
Accumulate 5000 points to unlock Akuma/Gouki.

Play as Shin Akuma/Gouki:
Acumulate 9999 points to unlock Shin.

Secret Modes:
Play single player mode to unlock secret modes such as survival and time attack.

In Game Reset:
While playing a game quickly press select and then start.

GameShark Codes

Unlock Gouki 
Obtain 5000 Points. 
  
Unlock Shin Gouki 
Obtain 9999 Points. 

Thanks to Revolution readers Ryan Max and abbu mania!