I find that when I am doing web development there are a few browser plugins that are very useful to me.
For Firefox I am using:
Firebug - Great for inspecting the HTML elements and working with CSS.
YSlow for Firebug - Developed by Yahoo! and gives timing and tips about page resources.
Live HTTP headers - Lets you inspect the headers that are sent to your browser.
For IE I am using:
Fiddler - "a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet"
I am always looking for other great tools to use. So what is everyone else using?