Test Drive Unlimited is a good game I recently came across. I like how they have seamlessly integrated single player and multiplayer, and also the fact that it is more expansive than any other car racing game. My only grouse is about the poor character modeling.
I like the Gran Turismo games, along with some of the Need for Speeds. Gran Turismo 3 is probably my favorite of that franchise, although they are all pretty good. I don't remember which Need for Speed I last played, but it was a really fun one, too. I think it was either Hot Pursuit or Undercover.