Cheats
Enter the following at the main menu:

Acura Integra	R2, R2, R1, R2, L2, L1, Down, Up
Acura RSX	R1, R2, Down, Left, Up, Right, Left, Right
Circuit Tracks	Down, R1, R1, R1, R2, R2, R2, Square
Drag Tracks	Right, Square, Left, R1, Square, L1, L2, R2
Drift Tracks	Left, Left, Left, Left, Right, R2, R1, L2
Sprint Tracks	Up, R2, R2, R2, R1, Down, Down, Down
Drift Physics	R1, Up, Up, Up, Down, Down, Down, L1
Ford Focus	Left, Right, Up, R1, R2, R1, R2, Up
Honda S2000	Up, Up, Down, Down, Up, Left, Square, R2
Hyundai Tiburon	Left, Right, Left, Right, Up, Down, Up, Down
Level 1 Performance Parts	R2, R2, R1, R1, Left, Right, Left, Right
Level 2 Performance Parts	R1, R1, R1, R1, R2, R2, Left, Right
Level 2 Visual Parts	Down, Left, Up, Down, R1, R2, R2, Square
Lost Prophets Car	Up, Up, Up, Right, Down, Down, Up, Right
Mitsubishi Lancer	Left, Left, Left, R1, R1, R2, R2, L2
Mystikal Car	Up, Right, Up, Up, Down, Right, Up, Right
Nismo Car	Up, Down, Up, Left, Down, Down, Up, Right
Nissan 240SX	Up, Down, Left, Right, Square, R1, L2, R1
Nissan 350Z	Right, Right, Down, Up, Square, L1, R1, L2
Nissan Sentra	Right, Right, Right, L2, L2, R2, R2, Up
Nissan Skyline	Down, Down, L1, L2, L1, L2, L1, Down
Petey Pablo Car	Up, Up, Up, Up, Down, Up, Up, Right
Rob Zombie Car	Up, Left, Up, Up, Down, Left, Up, Right
Subaru Impreza	Up, Down, Down, Up, L2, Up, L2, Down
Toyota Supra	R1, R1, R2, R1, R2, R1, L2, L1

Unlockables
To unlock the following complete Underground Mode:

All Accessories
All Body Parts
All Decals
All Driveterrain Packages
All Exhaust Packages
All Neons
All NOS Packages
All Paint Colors
All Rims
All Vinyl