I like dirpy. If you find a HQ song in a youtube video, its great. You have to look at a couple different videos to find the one thats the best quality but it works out well in the end
the quality is really really shit on youtube anyways, if you're just getting songs you might as well just download the actual song from torrents or whatever floats your boat
I've had nothing but bad results from this website, the song volume goes up and down randomly, the quality it shitty. I would use one of the other ones people have mentioned
I use DVDVideoSoft, it works well and I have no problem with it but if everyone's saying dirpy I might as well hop on the wagon, and maybe start using it : DIRPY
http://www.convertvideotoaudio.com/ provides free download and conversion of youtube files also you can upload and convert files from your computer also the service is free.