This one is pretty simple.
Easy to use interface to many free and commercial online validation and analysis services. Validate, analyze, view and inspect documents containing HTML, XHTML, CSS, RDF, RSS, XML, P3P, hyperlinks, metadata and many more. Check Section 508 and WAI compliance of your documents with many helpful services.
View Rendered Source
This one is also pretty simple, it seems useful, but since I usually download it and open it in Notepad++, I have no use for it.
Web Developer 0.9.3
This one I have had for as long as I can remember. I haven’t really used all the features–nor do I know them all lol–but it is quite useful.