Special Car:
To get special cars type in SPDSTR when you do a time trial.
Note: This will only work for 1 player games.

Other Cheat Codes:
Enter the following codes at the options menu:

CUP OF CHOICE = unlocks all cups.
I HAVE THE KEY = unlocks all cars and tracks.
THAT TAKES ME BACK = enables backwards mode.
NOTE: Where the spaces are you must hit the space bar

And more...:
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.)

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

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