Following codes by Jan Wargalla

Cheat mode:
Quickly enter one of the following codes at the title 
screen. The logo will zoom in with a sound to confirm 
correct code entry. To retry a code, enter another 
menu and return to the title screen.

View credits
Press Circle, Triangle, Circle(2), Triangle, Circle, 
Up, Right, Left, X. 

View victory FMV sequence
Press Circle, Triangle, Circle, Triangle, Circle, 
Triangle, L1, Up, R2, X. 

Enable all ten tracks
Press Up(2), Left, Right, Down(2), Circle, R2, 
Triangle, X. 

Enable all ten reversed tracks
Press Down(2), Right, Left, Up(2), Circle, L2, 
Triangle, X. 

Reverse mode
Press Left, Right, Left, Right, Circle(2), R1, L1, 
Triangle, X. 

50 Km/h AI bikes
Press Down(3), Circle, L1, Circle, L2, Down(2), X. 

Pocket bike mode
Press Up, Down, R2, L2, Down, Up, L1, X. 

Turbo boosted bikes
Press Up(3), Triangle, R1, Triangle, R2, Up(2), X. 

Night tracks
Press Up, Circle, L1, Down, Triangle, L2, Circle, Left, 
R1, X. 

Future vehicles
Press Up(3), Down, L1, R2, L2, R1, Circle, X. This will 
unlock a Jet Moto type vehicle in super bike mode, and 
a Surf Board in dirt bike mode. 

Bike only, no rider
Press Left(2), Triangle, Right(2), Up, Down, L1, R1, X. 

Rider only, no bike
Press Right(2), Circle, Left(2), Down, Up, L2, R2, X. 

Turbo enhancer

up,up,up,triangle.R1,triangle,R2,up,up,X