Gran Prix Ledgends is another one I really get a buzz outta from time to time.
The original game is pretty old and out of date, but there was and still is a very active modding mob out there. So much so that the the game was re-released by the makers in a cut down FREE version including some of the best mods available, called GPL 2004 demo here's a bunch of links the demo is first up... http://www.gpllinks.org/
If you have the original game, you can add the stock tracks to the demo to make a fully functioning new game Also, other addons work with the demo too...
It's based around the 1967 F1 season when the only onboard computer was the driver, the cars had raw power, treaded tyres and no wings for downforce...get set for some impromptue white knucke flying
The quality of the programming is high, as these things appear to be very close to the real deal. Immersion is galvanised by the incredible engine sounds from the Ferrari V12 to the BRM H16, they are all good.
A 1965 season set of cars is available (less power, skinnier tyres) & a multitude of tracks/circuits Dundrod Northern Ireland being one of the most challenging and very well done... http://www.jrpearson.homestead.com/
Also in the works are a set of 1967 sports cars, ie. GT40's and that kind of thing