Super Blurry Mode
To experience what Boss Games calls "Blurry Assed Graphics", enter the following button combination with the second controller during normal gameplay:
B, Left, Up, Right, Right, Right, A, Start, Start, Z, Right, A, Up, Z, Bottom-C, Start.

Note: Use the D-Pad for the directional commands.

Million Dollars Cheat
Want to get a million bucks? Enter the following cheat during gameplay (using the second controller):
B, Z, Z, B, Up on the D-Pad, Bottom-C, Z, Start

Top Down View
To player the game from a "classic" top-down view instead of the behind-the-car perspective, enter the following button combination during gameplay (using the second controller):
Up, Up, Up, A, Left, A, A, A

Get the Snowboard
Enter BUCKYB as your name to ride a snowboard.

Milk Truck
Top Gear Rally fans will be happy to see that the classic Milk Truck has made it into Stunt Racer 64 as well. Simply enter your name as "MOOOOO" to unlock it.