LibraryLookupThis is old but I needed to update mine and I had a hard time finding this page again.
This bookmarklet works for firefox. It adds a toolbar link so that if you are looking at a web page with a ISBN number you click to search the card catalog of your local library.
For example, you are viewing a book in amazon.com, click, opens a window to show the record in your local library.
I use it all the time.
Jon Udell: LibraryLookup homepage