Whenever you are on a youtube video page, simply insert 3 letters into your URL to download the video as video, as MP3, or other.The three letters are "pwn". You edit the YouTube URL and insert the letters "pwn" in front of "youtube.com", you'll get a url similar to this: http://www.PWNyoutube.com/watch?v=y7u7prP7sF0Follow that link you'll see more than 6 different links that will let you download a YouTube video as an MP3 or other audio file. Each link leads you directly to the page you need, eliminating the need to do any copy and paste of a YouTube url or video ID. The page is also updated as new services come online or go offline, meaning you'll never have to go find another solution if your favorite service stops working (happens more often than you might think!). For more information, see http://deturl.com . Deturl.com lets you download from many other sites also and includes other neat features, including a sweet bookmarklet with its own bonus features.

I've used ListenToYouTube but FetchMp3 definitely looks like it's much better with many more features.

Here's 3 more good sites for this task.

you can also use listentoyoutube.com

for downloading audio from any site, you can go to vixy .net

Alexandar,for music videos posted on YouTube and DailyMotion you can try FetchMP3. It does exactly what you're asking for.