Book Burro by Jesse Andrews was runner up in our software competition. I had seen mention of it before but had not tried it. I have just installed it and is is indeed very nice, as Liz Lawley, one of the judges, discusses.
It is a FireFox extension which overlays a small pane on a page with bibliographic data - like the Amazon page shown here. Currently it checks prices in several book buying sites, will look in selected library sites, or will interact with Worldcat.org to present a selection of libraries close to your zip code. You can configure it with the selection of book sites, library sites, and your zip code.
As Liz says, it is helpful, and from our point of view, it is great seeing WorldCat used in this way.