Recently I have been playing on implementing some extension for Firefox. When I started working on it, I was not aware on how far they have taken the use of metadata—I blogged before on metadata stores here. Actually, now you can query RDF data sources, which contain information ranging from your browsing history or bookmarks—to mention a few. But what makes it more exiting is that when you are writing extensions you can use java script to get that information stored by the browser using the RDF interface to do something useful. For instance, DISCUS is using it to provide on-the-fly analytics for the current page currently viewing. DISCUS will soon release a demo version of such a toolbar. If you want to read more about it, I would recommend the XULplanet.com.