If you want to use a web interface then try out these sites:
- www.youtube-mp3.org/ the parent site for the Chrome add-on
There are of course plenty more that are just a quick Google search away and also a bunch of add-ons for other browsers - this is simply to point you in the right direction (and if you didn't know it was possible beforehand, now you know!).
Enjoy downloading your favorite Rebecca Black tracks this Friday!