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:

Codes 
Unlock All Secrets
 Code     Effect 
240240sx Unlock 240SX 
350350z Unlock 350Z 
driftdriftbaby Unlock all Drift Circuits 
allmylvloneparts Unlock All Level 1 Parts 
seemylvl1parts Unlock all Level 1 Visual Parts 
allmylvl2parts Unlock All Level 2 Parts 
seemylvl2parts Unlock all Level 2 Visual 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 
givemenismo Unlock Nissan Sentra Nismo 
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.

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 
 
Extra Cars
Unlockable                  How to Unlock 
1998 Nissan Skyline GRT34   
  Select underground mode and complete at least the first 80 missions, then challenge Samantha. 
  Defeat her in medium difficulty & win the next drag race 
Eddie's Nissan Skyline GT-R 
  Get 3,000,000 style points in Quick Race 
Junkman's Honda Civic (Samantha\'s ride, the one that TJ messed up) 
  Get 2,250,000 style points in Quick Race 
Lost Prophets Car 
  Get 1,721,000 style points 
Melissa's Nissan 350Z 
  Get 2,500,000 style points in Quick Race 
Mystikal Car 
  Get 698,000 style points 
Nismo Car 
  Get 170,000 style points 
Petey Pablo Car 
  Get 533,000 style points 
Rob Zombie Car 
  Get 1,110,500 style points 
Samantha's Honda Civic 
  Get 2,000,000 style points in Quick Race 
Vortex Car 
  Get 46,000 style points 

Secrets 
Slower Enemies:
Note: This is recommended only for progress level 95 or higher 
(Underground Mode only). Enemies here are much faster than you, 
even if you're driving an optimized car. 
Even playing on easy mode doesn't make much difference. 
So change the turbo and nitro parts to stock. 
For some reason, your opponents will be much slower than you.
 
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.