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.
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.
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.
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.