Play as Shin (True) Akuma:
Highlight Akuma, hold Select and press Down, Right, Right, Down, Left, Down, Left, Down, Right, Right, Right. You should end up on Akuma - press any button.
Chun Li's old clothes:
Hold Select until her clothes change, then press any punch/kick button (without releasing Select).
To fight in Sagat or Bison's secret stage, highlight either Sagat or M.Bison and hold Start for 5 seconds.
Thanks to Revolution readers Herman Manher, Bill Wallace, and Eddie Alvarez!