CSSViewer

Linked from CSS Beauty I found, as described by one user, "the most useful extension since web developer toolbar". I'd potentially add Aardvark in there too, though the CSSViewer expands on Aardvark's territory by outlining an element and listing the styling that's been applied to it. The new CSSViewer is extra handy for those occasions where for example you spot a well-used font on a site and rather than trawl through the CSS matching up a style with a tag, you just activate the CSSViewer and hover over it. The one thing it could do with though is Aardvark-style activation via the right-click context menu rather than having to use the browser Tools menu. If you develop sites, get it now.