At the 'Options' screen select 'Enter Password' and enter the following case sensitive codes (without the quotes) to unlock the corresponding effect.

Unlock ALL Stages:
Enter 'SFO'

Unlocks ALL Tracks Up To Antartica:
Enter 'IGLOO'

Unlock China Track:
Enter 'LAMA'

Unlock ALL Normal Vehicles:
Enter 'WINNER'

Unlock Tiny Vehicles (except it's Wheels):
Enter 'QUARK'

Unlock The Special Best Vehicle:
Enter 'MADMAX'

Unlock The Special 2nd Best Vehicle:
Enter 'XAMDAM'