Here are two decent web services that you can use to rip the audio out of any Youtube video, and save it as an .mp3

Vixy.net

I found the first one, Vixy [ http://vixy.net/ ], over a year ago.  Generally it helped me when I wanted to grab a rare, b-side or unpublished song.  It seems these days it’s easier to find live than recorded work on Youtube, but this tool works like a charm.  They offer a Mac and Windows client to do the same from your own computer, but I always found the expiration dates annoying, and tend to prefer the web interface these days.  On the plus side, you can also rip the video if you want, in various formats.  Our second link is audio only.

Listentoyoutube.com

My girlfriend turned me on to this site: http://www.listentoyoutube.com/ which rips the audio at blazing speeds, and downloads nearly instantly.  I’d be curious to find out what they are doing so differently from Vixy to speed up the service.  If you just want a quick .mp3 of a song (or any video for that matter) you saw on Youtube, input the url at this site, and it will be done nearly instantly.

(By the way, I’m trying my 3rd post using WordPress post-by-email, it seems they don’t let my free account auto-include images by attaching them to the emails)