Jon Udell has extended his LibraryLookup bookmarklet so that it modifies an Amazon book page to show, right on the page, whether that particular book is available at your local library. He has a screencast showing how it works. And Jon explains this more in his most recent column at InfoWorld. I was thinking "this is cool" but I was totally blown away at the end when he shows how he used Amazon Wish Lists and the Amazon Web services API to create an RSS feed to remind him of books that had previously been checked out of his library and were now available.
This shows the incredible power of small applications joined together through Web services. Amazon's "hosting" a book service for thousands of distributed, disconnected libraries and neither Amazon nor the libraries know anything about it.
As an aside, this is all made possible by a Firefox extension called Greasemonkey. This is what I was talking about when I said I'd decided to start using Firefox over Safari. I haven't looked back and I'm very happy with Firefox.