Test Drive Unlimited 2 is a "racing MMO". I don't know how busy the servers are these days, but it meets the other criteria. I used to play it a lot when it came out.
Watch this for an idea of what it is like.http://www.youtube.com/watch?v=As9QOpWY8Dw
The game isn't amazing, but as the top comments point out it isn't anywhere near as bad as IGN makes it sound. I'd probably give it a 7.5/10 or so.
EDIT: If you like customization, it has quite a bit of that. You can tune your car and change wheel rims, add decals to your car and color the interior and exterior.