Start game as usual. Prior to entering each of the following cheat codes, go to the main menu, go to the Statistics sub-menu, go back again (pressing DELETE), and finally enter your desired cheat:

gimmesomecircuits - Unlock Circuits
gimmesomesprints - Unlock Sprint Circuits
gimmesomedrag - Unlock Drag Circuits
gimmeppablo - Unlock Petey Pablo
gotcharobzombie - Unlock Rob Zombie
havyamystikal - Unlock Mystikal
needmylostprophets - Unlock Lost Prophets
119focus - Unlock Car
893neon - Un1ock Car
899eclipse - Unlock Car
371impreza - Unlock Car
222lancer - Unlock Car
922sentra - Unlock Car
667tiburon - Unlock Car
334mygolf - Unlock Car
77peugeot - Unlock Car
777rx7 - Unlock Car
350350z - Unlock Car
111skyline - Unlock Car
221miata - Unlock Car
2000s2000 - Unlock Car
889civic - Unlock Car
228supra - Unlock Car
240240sx - Unlock Car
342integra - Unlock Car
239celica - Un1ock Car
973rsx777 - Unlock Car