This looks really interesting: Piggy Bank.
The basic idea is that this lets you add additional information about the web pages you visit to a database running on your machine using "Semantic Web" technologies. Now, the thing about the Semantic Web is that it adds meaning to web pages, instead of simply being text, so you can say things like "this page is about a movie, and it is rated PG".
On the developer side of things, this is built in a manner similar to Google desktop search in that the "server" is installed on your machine instead of a machine sitting on the other side of the internet. The GUI is completely built in Firefox, and uses LiveConnect to talk to the Java bit. It looks like it is really taking the "browser as platform" idea to the leading edge - with lots of AJAX thrown in for good measure.
I am looking forward to installing this and having a play.