The following bonus cars become available options when you Download and then beat the indicated amount of Challenge Series Events.

Unlock Battle Machine RX-7:
Win the first 30 Challenge Series Events.

Unlock Dominator Corvette Z06:
Dominate over 30 Challenge Series Events.

Unlock Speed Machine 911GT2:
Win ALL 60 Challenge Series Events.

At the 'Main' menu go to 'Options' and select 'Secret Codes'. Then enter the following codes exactly as shown to unlock the corresponding effect. These codes will only work once per profile.

Unlock $10,000:
Enter the code %%$3/"

Unlock NeedforSpeed.com Lotus Elise Bonus Car:
Enter the code-KJ3=E

Unlock Die-Cast Lexus IS F Bonus Car:
Enter the code0;5M2; (contains the number '0')

Unlock Die-Cast Nissan 240SX (S13) Bonus Car:
Enter the code?P:COL (contains the letter 'O')

Unlock Die-Cast Volkwagen R32 Bonus Car:
Enter the code!2ODBJ: (contains the letter 'O')