My fav is Grand Prix Legends, but it's too difficult to drive.But when you're able to control the car, the fun is too big to be described. But when you don't, GRRRR!You'll feel so nervous.
Try also Live For Speed
http://lfs.racesimcentral.com/
Or NetKar
http://www.nkzone.com/
in particulart NetKar is free and it's very very good