Simply the best. And latest version works with YT Rated 5 out of 5 stars
Best YT (and others) downloader of all the ones I tried. It is simple and unintrusive, and the author seems to constantly keep it updated so it works again quickly every time the the sites change.
September 2012: version 2.50 didn't work anymore with YouTube, but 2.052beta (https://addons.mozilla.org/en-US/firefox/addon/unplug/versions/2.052beta) does work.