I absolutely love the Web Developer Toolbar. One reason, that I ran into today, is that if I need to find out if a particular piece of (x)html is valid I only need to type data:text/html,<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html><head><title></title></head><body><p>sometext</p><img src='http://www.google.com/intl/en/images/logo.gif'/><p>moretext</p></body></html> and with two clicks of the mouse I can send it to the w3c validator. (The validator doesn’t have a form upload, so otherwise I’d have to write it, save it, and go through the upload steps on the site.) And since I have said data: url template bookmarked, it’s a total of 3 clicks and writing a minimal testcase for what I want to know–much easier than reading through the specs.
Note that the w3c validator does have bugs, so this shouldn’t be used for anything complicated or esoteric. Just the basics.
Nice trick!