Type these codes at the intro screen (the one that comes up before the main menu that says "Press Enter" -- right before your profile is loaded). If accepted, a small beep will be heard.
Code - Result:
needperformance1 - Unlock Performance Parts Level 1
needperformance2 - Unlock Performance Parts Level 2
gimmevisual1 - Unlock Visuals Level 1
gimmevisual2 - Unlock Visuals Level 2
regmebaby - Unknown Effect
ordermebaby - Unknown Effect
regmybank - Unknown Effect


Unlock Sponsor Cars
These codes are entered the same way and will unlock SPONSOR cars that can be selected by switching the car selection option to SPONSOR CARS from ALL CARS (using the right/left arrows on the menu).

Code - Result:
opendoors - The Doors GTO
gimmechingy - Chingy Navigator
tunejapantuning - Japan Tuning G35
davidchoeart - David Choe Corolla
shinestreetbright - Shine Street Lexus IS 300
wannacapone - Capone H2
wantmyd3 - D3 Eclipse


Unlock Sponsor Vinyls
These codes are entered the same way and will unlock sponsor vinyls. These are selected under the CUSTOMIZE menu, under VINYLS. Scroll all the way to the right and you will see SPONSOR as a selection.

Code - Result:
needmybestbuy - Best Buy
goforoldspice - Old Spice
gotmycingular - Cingular
gottahavebk - Burger King
gottaedge - Edge