Various Cheats:
At the first OPTIONS menu type:

cup of choice --This unlocks all cup races!
that takes me back --This enables backwards mode!
i have the key --This unlocks all cars and tracks!
i carry a badge -- Get Police cars
lone crusader in a dangerous world -- Get Nitro when you use horn
remote braking -- Use horn to stop opponents

*NOTE: Where the spaces are you must hit the space bar*

Other Cheat Codes:
To use these codes you must be fast enough to have your time entered on the high score screen (make sure to turn checkpoints on from OPTION screen.)

knacked - Reverse tracks
whoooosh - Nitros (press HORN while racing)
mjcim.rc - Tiny cars
sausage - Bonus cars

Note: After entering the codes make sure you save your game.
Otherwise, you will have to run this each time you play.