Various cheats
At the first OPTIONS menu type: 

Cheat code:             Effect:
cup of choice           unlocks all cup races
that takes me back      enables backwards mode
i have the key          unlocks all cars and tracks

     To  use the following codes, you must be fast enough to have time
entered  on the high score screen. Make sure you turn on 'checkpoints'
from OPTIONS screen.

Cheat code:             Effect:
knacked	                Reverse Tracks
whoooosh	        Nitro (use HORN)
mjcim.rc	        Tiny Cars
sausage	                Bonus Cars
cup of choice	        Unlock All Cup Races
that takes me back	Backwards Mode
i have the key	        Unlock All Cars/Tracks