First Person Mode:
Pause the game. Then hold L and press Y, B, Y, B. Repeat the code to disable its effect.

Turbo Mode:
Pause the game. Then hold L and press Y, B, X, X. Repeat the code to disable its effect.

Welsh Mode:
Pause the game. Then hold L and press Y, Y, Y, Y. Repeat the code to disable its effect.

Enable "Real Controls":
Pause the game. Then hold L and press Y, Y, Y, B. Repeat the code to disable its effect.

Squeaky Voices:
Pause the game. Then hold L and press Y, X, Y, Y. Repeat the code to disable its effect.

Mutant Mode:
Pause the game. Hold L and press Y, B, X, X. Repeat the code to get Mutant modes 1,2, and 3. Repeat again to disable the effect.

Thanks to Revolution reader Class M!