Fart Cheat:

While playing as the Doctor Hold L + R + Left on the +Pad + A

Get Matrix Mode (Slow Motion) Cheat:

While playing as Max Hold R and Press Up on the +Pad 4 times.

Kurt In Boxers Code:

While on the Main Menu, Hold L + R and Press X, X, Y, X.

Indy 500 Camera View Code:

During the game Pause, then Hold L + R and Press B, A, B, A.

Bonus Art & Audio Files:

Put the MDK2 disc in your PC CD drive, for some mp3s, wavs, and rendered, comic, and concept artwork.

Full Pause Screen:

While paused Press X+Y.