Roll Your Own Custom Bookmarklets In Firefox, part 2 - page 2
Or right-click on this link and choose "Copy Link Location": Find back-refs. Now you'll be able to invoke the bookmarklet on any page and it will tell you links to that page.
Here's a bookmarklet that translates a page from another language using Babelfish, provided you know the appropriate two-letter code: Babelfish translate.
A frequently requested feature for Firefox is a button to go up one level from the current page. But you don't need a button if you just make a bookmarklet: Up one level.