This is the easiest free one I’ve used. I recommend it, specially if you don’t have a fast computer, which makes YouTube choppy (I recommend downloading their little software — it’s a LOT faster than the site!) Sometimes when converting to AVI they’re not the best quality, though.
http://clipnabber.com/ (This one also grabs from other sites.)
(There are also many more — just type “youtube download” into google!)
I use something called Orbit Downloader which can also download almost any streaming media (not all!) and other things as well. (I’ve scanned it for viruses and spyware, and it’s clean!) Although now it won’t integrate properly with Firefox 3.01.
If you know any others, please post in the comments!