Apart from Nascar 2003, which may be the only serious racing sim for the Mac (but runs great on today's hardware though), and the far-less advanced F1 CS 2000, there's also Toca Racing Driver 3 (Race Driver 3 / DTM 3), which is fairly ok if not quite up there with the newest.
Thru Cider, you can also run pc-based racers like Grand Prix Legends and Need for Speed Underground/Most Wanted/Carbon directly on OS X... None of them are free, though, and I don't know of a real, complete Mac sim which doesn't cost some $.