The PageSpeed tool included with Firebug is quite helpful. I found one serious flaw however: the MINIFY HTML suggestion. They give this suggestion for most pages also providing the so called improved version of a page (white spaces removed and so on). I used the "improved" code for 3 of my pages. Then I had to work 2 hours to get my pages back. Here's why: this tool doesn't seem to differentiate between html versions, thus removing all closing tags that are mandatory in XHTML (with proper DOCTYPE in place), all quote marks (<a href="etc"> becomes <a href=etc>), and other changes resulting in over 1000 validation errors for each of these 3 pages.