I got this lead from Ben Toth. This is a brilliant application.
It doesn’t work with many sites, but it does on Amazon.com. When you load a page with an ISBN number Book Burro recognises this and automatically compares prices over a variety of book stores. In additon, it has agreements with some libraries to show stock.
As you will see on the screenshot, Amazon is by no means the cheapest and also that the book is available in Earlham and Manchester Colleges. Not that that does me much good as I’m based in Cardiff – still it’s the principle that’s important.