http://www.bing.com/search?q=youtube+downloader+greentree+applications&qs=n&form=QBRE&pq=youtube+downloader+greentree+appli&sc=0-34&sp=-1&sk=&cvid=21040F5F67A24A049E8BF171B11AF070Top link. Should mention GreenTree Applications. They developed it. I would link directly to it but as Speck said, Fizzer would probably delete the link. We wouldn't want to promote pirating music, would we?
It's perfectly safe to download, though you're welcome to scan it with your antivirus software first.
It allows you to download from loads of video sites and convert to loads of file types, including MP3.