Firefox/Chrome with Greasemonkey: BookInfoLine is a lightweight Greasemonkey script that adds in instant and automatic price comparison to book sellers' websites. Know immediately if a book is cheaper somewhere else.
Visit one of the supported booksellers and BookInfoLine pops up a small yellow box in the upper right corner of your browser window. Click on it and it expands to display the prices on all the other supported book sellers. BookInfoLine works on Amazon, Abebooks, Half.com, eBay, Alibris, Barnes & Noble, The Strand, Google Shopping and more.
Visit the link below to read more and grab a copy for your browser — BookInfoLine works wherever Greasemonkey does.