Atube Catcher is a free application built for xp and above. It is aimed at video downloading from sites such as youtube. It also allows you to convert video & music to whatever format you want. It does not even have to be youtube that you download from.
You can also convert AAC, FLA & Shockwave formats as well to what ever stuff you want but not the other way.
I had been searching for ages for a video/music converter and ways to download stuff from youtube & was such a happy person when somebody told me about this application.
Another feature is that you can record exactly what you are doing on the screen, which is brilliant for tutorials. I have come across millions of alternatives that cost a lot more than what they are worth.
I would give it 10 out of ten but it is not available for Mac or Linux so I have given it an 9 out of ten instead.
Hope this helps :)