I'm a big fan of Quicksilver, an application launcher for the mac (that's really an understatement, because you can do a so much more than just launch applications, but I digress).  So when I came across this Firefox plugin called Ubiquity the other day, I became immediately excited.  Ubiquity is like Quicksilver for the web.  It allows users to tell the browser, through language, what they want to do.  What does this mean exactly?  Check out the video below:

This plug-in allows you to validate HTML and CSS of the current webpage on-the-fly using the W3C validation engine and the Firefox plugin called Ubiquity.  You can download Ubiquity here.  Once installed, you can simply goto command mode (alt-space by default) and type either: 'validate-html' or 'validate-css'.