Book Burro started as a Greasemonkey extension to help me find books in gradschool. It evolved to extensions for Firefox and Chrome.
Book Burro was a Web 2.0 extension for Firefox. When it sensed you were looking at a page that contains a book, it overlayed a small panel which when opened lists prices at online bookstores such as Amazon, Buy, Half (and many more) and whether the book is available at your library.
It leveraged APIs when possible, but resorted to scraping book stores and libraries. Additionally users could add scraping rules to check their library and share it with the rest of the community.