Is there a car rental price "notifyer" - deal detector, etc...

[MermaidMom]

I'm thinking of something like Orbitz's Deal Detector for air fares where you set a price and then get an e-mail that they found it. I'm guessing they run a saved search every day.

Does anyone offer something like this for car rental rates? I can't seem to find it on the main sites - Orbitz, Travelocity or Expedia but maybe there is a lesser known one that does this?

Thanks!