Ruud van Gaal is building an open source racing simulation called Racer. GPL was his inspiration. He says, "this IS an attempt to get something so open that a GPL2-like program would be possible by others to make."
Ruud has done an amazing amount of work and has documented it very well on his Racer Free Car Simulation site.
You can download the current development version, including some tracks and cars at Ruud's site. You can also convert SCGT tracks (though this isn't a simple click and go process at this point), and Ruud's made available some files that make it easier to convert the modern version of Monza (originally from F1 2000).
Racer is very much a work in progress, and there is still an enormous amount of work to be done. Things like tire tempuratures, AI, multiplayer, damage, skid marks, and even different friction and drag coefficients for grass have yet to be developed.
However, it is driveable, and after a bit of tinkering I was able to get the latest version up and running quite well on my Thunderbird 900/Voodoo5.
After tweaking some of the files for the Ferrari 312, I got it handling passably well, at least at low speeds. It's a hoot to throw it around the slow corners of Carrera, although it's far to eager to get into a "zippy" if you crest the hill too fast.
I think Racer's potential is enormous, particularly because of its open architecture and Ruud's open-source philosophy. Potentially, this is a sim that can turn out however the sim community wants it to. If someone wants a new feature, they can just add it themselves, and make it available or pass it back to Ruud for incorporating in the release version.
Potentially, it is possible to simulate any cars, any tracks, from any era, without constraints put on the project by potential marketing issues. Amazing!
Look at all the effort now going into development of tracks, cars, utilities, and other enhancements for GPL. Now imagine if even half that effort were going into a sim like Racer!
Get it at Ruud's Racer Free Car Simulation. Drive it around a bit, and then imagine what it could be. And think about how you might help make it become that.