rFactor is an awesome one, it's $40 though, so it isn't cheap (
www.rfactor.net)
And Live For Speed S2 is probably the best ($50) (
www.liveforspeed.net I think)
These are the two I use, there is also a free one out called Racer. (
www.racer.nl)
Racer is OK, just it is designed to have extra cars added, so if you aren't very comfortable with copying and pasting files into directories, then try the other ones