For Mac OS X 10.5 or higher. If you use Mac OS X Tiger (10.4), you can download it on Tube 2.3.
The easiest way to download a video from youtube is to get Youtube, a light application that allows you to do just that one click.
The tutorial is compatible with Safari and Firefox. When you surf the youtube you will see a new button in the browser, click it and you will be able to download featured videos about a seconds.
You can download the video as an audio track as MP4 and save it as MP3.
The tube also provides a download manager so that you can add videos to the download list and you can download them whenever you want.