GTR 2002
http://www.gtr.kickassgamers.com/page.php?id=7
Below the download section they talk about the game. It sounds pretty neat, but you need the retail game first. Impressive mod. Also states that EA has hired them on for future games iirc.
Racer :
http://www.racer-xtreme.com/
You can get their latest beta there, and download a bunch of custom stuff. Looks very promising and the physics are neat, but beware that it is VERY much still in beta and could be unstable. It crashes or restarts one of my systems almost every time i run it. Fun little game thouhg, might want to check it out and keep an eye on it.
Also i hear the new Need For Speed coming out is supposed to be very promising, but again, i still haven't heard of a game as badass as MCO for customizing your car, and the physics your engine / suspension changes make...