Two new sites help you get the lowest price for your hotel room:
Tingo - prepay for your room thru this site and get automatic refunds if the price drops.
BackBid - tell the site the price you booked and your criteria, similar hotels will bid lower prices or more amenities to get you to change your booking.
And Autoslash monitors your rental car booking with Hertz, Thrifty and Fox to find any possible savings.
More details on these and other services here and here.