Go to the first OPTIONS menu and 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 -- Drive police cars
lone crusader in a dangerous world -- Press horn for nitro
remote braking -- Press horn to freeze 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.

Also, if you use the Nitro code don't try it until you're in second or higher gear. Otherwise you'll spin out.

Special Car:
To get special cars type in SPDSTR when you do a time trial.
Note: This will only work for single player games.