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.
Stumbleupon is a brilliant downloadable toolbar that beds into your browser and gives you the chance to surf through thousands of excellent pages that have been stumbled upon by other web-users