I will be in Europe for 3 weeks (Germany from May 2-16 and France from May 16-22).

I tried to use Priceline's Name Your Own Price but it doesn't seem to be available for car rentals Europe. Booking through their normal process, I can get the following prices for a compact car:
May 2 (Munich) - May 16 (Koln/Cologne) = ~$600
May 16 (Paris) - May 22 (Paris) = ~$350

I think I've been spoiled by Priceline's bidding on getting great hotel deals so now I'm expecting the same from my car rentals. Besides, Priceline/Expedia/Orbitz/Hotwire, what are the other good sites out there for finding a car rental in Germany/Europe?

Thanks!