I've just downloaded this racing sim, and it's by far the more realistic I've seen. It reminds you somehow to airplane simulators. To start the engine you have to mouseclick on the top switch in the cockpit, and then in the middle one. You can get out of the car and walk around the circuit.
The tyres wear out, have graining... realistically and you can actually see flatspots, blistering... in the surface texture. If you've got a force feedback wheel you will feel the flatspots, curbs... in the most realistic way I've found in a race sim. If you read the credits, you will find out it was developed by F3 drivers and engineers. Oh and it has an exclusive sound system that calculates each piston fire and the exhausts shape!
To download it:
http://www.netkar-pro.com/
I'm still 2.9 seconds away from the record, it's 1:33.6 on the track and car of the free demo.
Some screenshots:






