Lily is a Firefox add-on that allows users to create rich, complex applications in a web browser. With Lily it's possible to make programs that mashup the web or the desktop, visualize and animate data, modify webpages, play music, or connect to world outside the computer.
TiddlySnip is a Firefox extension that lets you use your TiddlyWiki as a scrapbook! Simply select text, right click and choose 'TiddlySnip selection'. Next time you open your TiddlyWiki file, your snippets will be there, already tagged and organised.
FireStatus is a Firefox 3 extension that allows you to send status updates, notes and URL links simultaneously to multiple social networks and services, like Twitter, FaceBook and FriendFeed. It also displays updates from these services in notification popups, for a more "live" communication with your friends.