Jesse Andrews has built Book Burro, a Firefox extension to display competitive pricing information for books. The information is presented in an inobstrusive, semi-transparent popup window.
In order to install this you will need Firefox and Greasemonkey. Greasemonkey allows your browser to run client-side scripts against any web page. Think of it as local post-processing after the page has been downloaded.
As an aside, it is pretty cool to see what Jesse has done with less than 300 lines of script -- he's making asychronous calls to multiple web services, capturing the results, and creating a nice UI.