I do a fair amount of 'web development', meaning I tend to write things that are viewable via a web browser. Whether they're posted on my blog, or on other sites I sometimes maintain, generally my preferred 'user interface' is a web browser.
I've been using a plugin for quite a while called "Web Developer Extension". It's a set of tools that integrates tightly with Firefox and lets you do all the things a web developer needs to do to make sure his or her application is displaying properly.
The most useful feature I've found is 'Outline block elements' and 'Display ID and Class Detail' - these functions change your displayed page and draw lines around all your block level elements (such as 'div' and 'table' and 'span'), and can also label them with what class and ID they are.
When dealing with multiple nested CSS elements, this sort of display can save HOURS trying to track down what element belongs to what container, particularly when working with content management systems like Movable Type.
The plugin is non-intrusive, and is only triggered when you select it off the menu. I can't imagine doing web development without it.
I can't believe I haven't paid attention to this before! (The plugin, not you.)