Turbo mode:
Pause the game, hold L and press Y, B, X, X.

Welsh mode:
Pause the game, hold L and press Y, Y, Y, Y.

Enable 'Real Controls':
Pause the game, hold L and press Y, Y, Y, B.

1st person mode:
Pause the game, hold L and press Y, B, Y, B.

Squeaky voices:
Pause the game, hold L and press Y, X, Y, Y.

Mutant Mode:
Pause the game, hold L and press Y, B, X, X.
Repeat this Code to get to mutant mode two.