There's quite a few youtube plugins for Firefox.
sudo apt-get install firefox
Fire up Firefox .. then go to Tools>Addons>Extensions
.. in the search box enter easy youtube video downloader
and hit enter.
Click the Install
button next to "Easy Youtube Video Downloader 6.6".
When installed .. restart Firefox.
Now when you visit any Youtube page .. there will be a "Download
" button under the video where you can choose the download format
I suggest you download them as .flv or they might get quite large
.. but if it's music, you can download them as .mp3 (ie. without the video) if you wish.
There's a few for Chrome/Chromium too .. but I've not tested them:http://www.chromeextensions.org/?s=youtube&scat=all
This seems to work in Chromium (just tested it):http://www.chromeextensions.org/utilities/ultimate-youtube-downloader/
for video it just works .. for mp3 conversion it takes you to an external site, where you click the download link