These are some Game Shark codes:-

Extra cars 800552780014 
Extra tracks and cars 800564560114 
Unlock everything 800564560106 
Freeze lap timer/place first 800659c00000 800659c40000 

Change Car Handling and Dynamics:
These codes need to be entered as your name for a season race - you can then quit and access the cheats in all modes:

No collisions and fast acceleration - enter name as 'Banzai' noviscous - This gives lets sand, mud, water, etc not to slow you down. Game is in fast motion - enter name as 'radbrad' 

Heavier cars - enter name as 'stone' 
    Make cars Lighter 'feather' 
    Lower Gravity 'float' 
    Reduce tire friction ' spinner' 
    Huge Tires 'fat tires' 
    Hover cars 'no wheels' 
    No car 'wheels' 

ACCESS CODES:
    beat rookie season 'vet me' 
    beat veteran season ' im a pro' 
    beat whole games and receive all cars trucks and tracks 'weeoo'