I like Goodreads' site for lots of reasons - seeing what my friends are reading, reading book reviews, keeping track of books I want to read in the future, finding out about author readings in my area - and now this:
"Find the best book prices!
Does the global financial crisis have you down? Goodreads just made book buying so much smarter and cheaper. Now you can find the best price for a book with one click! Search for any title and then click "compare prices" on that book's page. We are working hard to help you find new, used, and collectible editions of the books you want. You can also prioritize your preferred booksellers!"