Alternate costumes
Successfully complete arcade mode with any character, then save the game. Highlight any fighter, then hold Start and press an attack button at the character selection screen to change their costume color.
Switch screen resolutions
At the Capcom logo, hold L + Start to switch to a low resolution mode. To switch back to high resolution interlaced mode, hold Start + X at the Capcom logo.
Play as Gill
Successfully complete the game with all characters, then save the game. You may use continues, but not change characters. Then at the character selection screen, highlight Yang and press Down or Yun and press Up.
Extra options
Successfully complete the game with fifteen different characters with the system set to "Normal". This will not work if it is set to "Original". This will unlock an extra options menu and Gill.
Taunt
Press L + R to taunt your opponent.