If you are a Web developer, and you are not using Firebug, stop what you are doing and go install it immediately.
I first heard about Firebug on Rick Strahl's blog a couple of months ago, but in the flurry of work I promptly forgot it. A couple of days ago a friend was listing the reasons that why Firefox pwns IE and Firebug was the first.
Firebug is an add-on for Firefox. It allows you to inspect the details of the currently loaded page. You can visually inspect the DOM, execute JavaScript snippets on the fly, and more. I'm thoroughly impressed.
My feature du jour is the way it displays the CSS box model for the currently selected element.
I'm not going to repeat the list of features here, you can get that from the Firebug page. If you have not tried it, go check it out.