5 years later, I'm still using Fullrip.net and YouTube-mp3.org to download from YouTube and similar sites, but I've just found these three others that work just fine when those two are being temperamental (not often, but happens):
OnlineVideoConverter - different audio formats/qualities
YouTubeInMP3 - reads ID3 tags
Both good, different options and languages too.
Online converter - converts video, images, audio and documents, from and to a lot of different formats from different sources, but doesn't support YouTube.
Edit2: and also, these works too,
Converto.io - mp3 or mp4, different qualities.
Convert2mp3.net - Convert Youtube, Dailymotion, Vevo, Clipfish and MyVideo videos online to MP3, MP4 and more formats.