Key, do not use the youtube downloaders, terrible sound quality. Use torrents, download the vuze software at www.vuze.com then go to http://www.kickassunblock.info/ and look for your favorite artists.
100% Free - I have this as a Firefox add-on, or you can just paste a Youtube link at their site. Whenever I open a Youtube video there is a button that says, "Download mp3" with an arrow. The dropdown menu allows conversion to CD Quality, 320 Kbps. I've used these mp3s DJing in large dance clubs. The quality is perfect.