Get Tube is compatible with Safari and Firefox. While you surf Youtube, you'll see a new button in the browser, click it and you'll be able to download the featured video in a matter of seconds.
You can download the video as mp4 oronly the audio track and save it as mp3.
Get Tube also features a download manager so you can add videos to the download list and download them when you'll want.
For Mac OS X 10.5 or higher. If you use Mac OS X Tiger (10.4) you shall download Get Tube 2.3a.