All GT2 Circuit cars:
At the Teams/Event screen in Championship mode, press Z, Right, Z, Z, Z, B, C-Down, A, Right, Start on Controller 2. This should unlock all GT2 cars. You will not gain experience points by racing, however.

All cars and tracks:
Start Championship mode and enter RACES THE BEST as a name. Now press Left, Right, Right, Left, Down, Up to enable all cars and tracks.

Pink cars:
Enter your name as IGN64 in Championship Mode. All cars should be glowing pink.

Falcon Interceptor:
Beat the game with a gold trophy in all events. Return to the Event Selection screen and pick the Novice cup. Beat the Falcon Interceptor in both races. It should now be unlocked under Boss Racing.

Thanks to Revolution readers Bruce Howard, Bardley Fann, Anthony Medico, Erick, and Philip Wren!