Need for Speed Underground [cheats] Start game, go to the main menu, go to the Statistics sub-menu, go back again (pressing DELETE), and finally enter your desired cheat:

240240sx - Unlock 240SX 
350350z - Unlock 350Z 
allmylvloneparts - Unlock All Level 1 Parts 
allmylvl2parts - Unlock All Level 2 Parts 
239celica - Unlock Celica 
gimmesomecircuits - Unlock Circuits 
889civic - Unlock Civic 
gimmesomedrag - Unlock Drag Circuits 
Slidingwithstyle - Unlock Drift Physics in all the game modes 
899eclipse - Unlock Eclipse 
119focus - Unlock Focus 
371impreza - Unlock Impreza 
342integra - Unlock Integra 
222lancer - Unlock Lancer 
allmylvloneparts - Unlock Level One Upgrades 
allmylvl2parts - Unlock Level Two Upgrades 
needmylostprophets - Unlock Lost Prophets 
221miata - Unlock Miata 
334mygolf - Unlock Mygolf 
havyamystikal - Unlock Mystikal 
893neon - Unlock Neon 
gimmeppablo - Unlock Petey Pablo 
77peugeot - Unlock Peugeot 
gotcharobzombie - Unlock Rob Zombie 
973rsx777 - Unlock RSX 
777rx7 - Unlock RX7 
2000s2000 - Unlock S2000 
922sentra - Unlock Sentra 
111skyline - Unlock Skyline 
gimmesomesprints - Unlock Sprint Circuits 
228supra - Unlock Supra 
667tiburon - Unlock Tiburon 
2000s2000 - Unlock Updated Honda 2000 
899civic - Unlock Updated Honda Civic 
342integra - Unlock Updated Honda Integra 
973rsx777 - Unlock Updated Mazda RX7 
239celica - Unlock Updated Toyota Celica 
228supra - Unlock Updated Toyota Supra 

Unlimited Money Without Racing 
First you must obtain the "Nissan Skyline" then you have to atleast unlocked 3/4 of all the performance upgrades(Note: Having all of them will allow you to make more money)Get rid of the performance parts that upgrade the Skyline's top speed and change the turbo package back to stock. The performance stats should read "S, S, 3, S, S, 3, 3, 3, S". (Or it might be different if you haven't obtained all performance parts) Then trade it in for the Mazda Miata getting a rebate of $12727 afterwards re-equip everything onto the Miata then trade it in again, this time for the Nissan Skyline getting a rebate of 119 dollars. Then just repeat the whole process again and again each time getting $12846 each time so you can buy all the Visual Parts and Performance Parts quickly. 
-from EternalBlade 


Unlock All Visual Upgrades 
All Accessories - Complete Underground mode. 
All Body Parts - Complete Underground mode. 
All Decals - Complete Underground mode. 
All Paint - Colors Complete Underground mode. 
All Rims - Complete Underground mode. 
All Vinyl - Complete Underground mode 
 
