http://www.zemanta.com/
Zemanta plugs into the major blogging platforms (WordPress, blogger, TypePad) and your browser to suggest related content (pictures, tags, links, etc) as you type a post. It can then auto-insert the results into your post (add links, pics, etc). Useful.
http://www.getfirebug.com/
Firebug is a powerful debugging tool for web development. It's primarily a Firefox extension with a lite version available for IE, Opera and Safari. It allows you to debug, inspect, edit on the fly, and much more.
https://addons.mozilla.org/en-US/firefox/addon/1117
FoxClocks lets you keep an eye on the time around the world - or just your local time - by putting small clocks in your statusbar. Hat tip James Farmer.
https://addons.mozilla.org/en-US/firefox/addon/3899
HackBar gives you a really big address bar with a load of extra functions to test sql injections and so on. Useful for testing. Not currently 3.0 friendly.