http://www.midnitechallenge.com
VERY addicting, and you can when some crazy prizes. READ THE GUIDES WHEN YOU SIGN UP. They help a lot, and they're relatively short.
Note: For the first game you play, mess around. Mod your car from the beginning, get a -----ton of money, whatever. Just learn the game inside and out.
Then, starting with your second or third game period:
Don't mod your car or collect a -----ton of money until the very end. Spend 90% of the game leveling up and keep your net worth as low as possible (gives you lots of experience quickly), then the last week and a half or so (every game period is like 28 days then it's re-set) buy a good in-game car (Shelby Cobra or Honda NSX-R, or a Porsche) and do everything you absolutely can to it. And get your net worth as high as possible.