At the intro screen type the following codes. The intro screen is where it says; "CLICK to Continue" right after the intro movies and BEFORE the main menu. If entered correctly, a message will come up letting you know what you have unlocked.
Code - Result
castrol - Unlock Castrol SYNTEC Ford GT in Bonus Cars
burgerking - Unlock Burger King Special Event in Challenge Series
To get an extra $10,000 in career mode, have a Need For Speed Underground or Need For Speed Underground 2 career game saved on your hard drive. After the prologue you will have $40,000.