All cars and tracks:

Simply enter multi-player mode and select the "Peer To Peer" option, but race alone. You will be able to select any car on any track.

All cars:

In name selection type in "allporsche"

Remote control cars:

Make a new profile and name it Gulliver. If you type it right you will hear a special sound to confirm. This will make all the cars little remote controlled vehicles. To disable this cheat you must restart the game.

Unlimited money:

Purchase a used car and choose an "Overall" repair. When completed, the value of your car will be higher than the purchase price and the cost of the repairs. Sell the car, purchase another used car and repeat as needed.

Cheat Mode:

To use these cheat codes, goto the main menu and click "Create Player" and type in the cheat for the name. Click "Done" (you'll hear a little confirmation sound). Then choose your regular profile and play the game!

Gulliver - The Porsches are all shrunk down to the size of radio controlled racers and have little antennas.
Smash Up - Using the Skidpad, Destruction Derby style racing nothing better than that!
Dakar - Tired of Street Racing? Changes car physics and tracks to Rally style.
Fuzzyfuz - Enables cops in quick race (cops may not appear on all tracks: i.e.-Monaco).
yraGyraG - Enables all cars to have 993 physics.
fetherw8 - This cheat code makes cars way heavier.

Hints:

Do not, repeat, do not, brake while in the midst of a turn, particularly when driving older automobiles. The EA version of an antique Porsche has a tendency, and probably rightly so, to seriously rock your world if you do, sending you into a drift from which you may not recover. Instead, brake early, turn smoothly, and accelerate on the exit.

There's no better way of ridding oneself of an annoying police officer than by nudging him into a passing motorist or solid roadside object.

Alternate routes are not always shortcuts. If you find yourself consistently slow on a given course, check out all the routes available to you before resorting to a different setup.

Go with the "front bumper" camera for the best results. Here, the program doesn't have to pump out cockpit and driver animations, so the frame rate is generally faster. Also, you get a better sense of the speed at which you are traveling, so you'll be better prepared to slow down for upcoming turns.

Torque rules. If you find yourself in a section where you can use a higher gear with lower RPM or a lower gear with high revs, go with the latter every time