This web link allows you to convert anything from youtube and places it in your itunes. To begin go to youtube and find the music video or song you would like. Listen to it to make sure the quality is good. Then copy the URL and paste it in video to MP3 and click convert song. After a few seconds it will have you click finish converting and it will convert right onto your computer and into your itunes.
This program allows you to convert purchased music from itunes to mp3s so you can use them in your own videos, audio recordings, etc. It costs $20, but so far I think it's worth the money. It's fast and really easy to use.