The other day I was making a cd for a friend, and one of the tracks she'd requested was from a youtube link, and for the life of me I couldn't find an mp3 on any of the sites I usually frequent. This is something I've run into quite often, but this time, I remembered a link I'd seen recently on twitter: Dirpy.
It's got quite a few options, including not just pulling the audio, but also pulling a time range of the audio and editing the id3 tags. Of course, it's limited to an upper quality limit of 128 kbps, but then again, it's youtube, so the source quality is such that you're not going to get blood from a stone.
Also (potentially) useful is that it provides direct download links to the video. Sure, there are plenty of other ways to grab youtube video, but it's nice to have these functions all in one place.