When the Nordschleife is open to the public, you can take any street legal vehicle (car, motorcycle, bus, van), but the common car rental companies (Avis, Hertz, etc.) do not allow their vehicles to drive on the track. There are lots of stories where they (Avis or whoever) somehow found out and charged the driver a 2,000 Euro penalty. Check out Ben Lovejoy's page for more car rental info: http://www.nurburgring.org.uk/carhire.html

Anyway, if you don't have your own vehicle, or want to rent one for some other reason, check out Rent-Racecar (www.rent-racecar.de/gb). The owners - Theo and Heide - are very nice and just a pleasure to deal with, and they both speak English very well so there's no language barrier (assuming you speak English...).

They have a huge selection of cars including a BMW 318, BMW 325, BMW M3 (both E36 and E92), Nissan 350Z, and Porsche Cayman S. Check out their website for pricing details, and send email for vehicle availability.

I recently rented the BMW 325i E36 race car and had a great time. I had to change my reservation thanks to bad weather and they were very helpful in lining up another car for me on short notice. Also, they allowed me to take one extra lap in a 2nd car (a non-race car with back seats!) so I could take my wife and kid along for one lap of the Nordschleife.

I'm already looking forward to the next time I can take a trip out there, and I would gladly give my business to Rent-Racecar again.