At the main menu, press and hold L2+R2, then press Up, Up, Up, Square, Square, Square.
Car Showroom
At the Title Screen, press and hold L2 + R1, then press TRIANGLE, TRIANGLE, UP, UP, LEFT, RIGHT, DOWN, DOWN, X, X. After you enter the code correctly, the screen will go black and you'll be able to select any car or boss vehicle to view. Not that helpful, but, if you really want to get an upclose view of the Tank O' Death, you can do so with this cheat.
Extra Cash Bonus
During the race position your vehicle on a power-up icon that always reappears as cash. The icon will keep reappearing and you will keep picking it up. Just be sure to finish the lap before time runs out or you will lose all the cash that you picked up. This also works with icons that always reappear as nitros.
Infinite Cash
At the main menu (the one that lets you choose your car, your character, equipment, etc) press and hold L2+R2, then press Left, Left, Right, Right, Square, Square, Circle, Circle. You should hear a sound if you did the code correctly.
Note: Your money won't visibly increase, but it won't decrease when you buy cars, equipment, etc.
Infinite nitros
Pause the game, hold R1 + R2 and press Down, Down, Down, Circle, Circle, Circle.
Infinite weapons
Pause the game, hold R1 + R2 and press Left, Up, Right, Down, Square, Triangle, Circle, X.
Invincibility
At the main menu hold R2+R1 and press Up, Left, Right, Down, Triangle, Circle, X.
Small Cars
At the main menu, press and hold L1+R1, then press Down, Down, Down, Circle, Circle.
Total Chaos
At the Title Screen, press and hold L2 + R2, then press DOWN, FORWARD, DOWN, FORWARD, CIRCLE, CIRCLE, CIRCLE. If you enter the code correctly, you will be taken to the Total Coas screen and you'll be able to make selections from playing the game from the new screen.
NOTE: You can ONLY turn the Total Choas mode off by resetting your system.
Voodoo Car
At the main menu, press and hold L2 then press Left, Right, Down, Up, Square, Circle, X, Triangle.