I haven't used any of these yet, so I dont know if they work. But I did a search for cheats for the PS2 version and this is what i found:
Enter the following codes at the main menu:
Effect - Code
Get Free Parts: X, X, Triangle, X
Unlimited Nitro: X, Circle, Triangle, Triangle, X, X, Circle
Unlock all Circuit Tracks: Down, R1, R1, R1, R2, R2, R2, Square
Unlock all Drag Tracks: Right, Square, Left, R1, Square, L1, L2, R2
Unlock all Drift Tracks: Left, Left, Left, Left, Right, R2, R1, L2
Unlock all Sprint Tracks: Up, R2, R2, R2, R1, Down, Down, Down
Unlock Drift Physics in All Modes: R1, Up, Up, Up, Down, Down, Down, L1
unlock everything rating: ---
go to main menu and press up down up up up down circle squareand up then turn off your game and turn it back on you will have unllocked everything
Hint: Money Mayhem
Play through Underground Mode as you would usually do, unlocking a good amount of cars and performance parts. This is just your initial investment (or 'seed money') for the trick. Save the game. Then buy and modify the VW Golf to the maximum and trade it in for the Honda S2000 for a gain of about $1500. Take the Honda to the garage and demodify it (strip the parts) and trade the Honda for a Skyline (and a loss of about $3000). With the Skyline, you can now trade for the VW Golf for a gain of about $10,000. You can repeat this car laundering until you have enough cash for your dream machine