Dirpy allows you to easily rip the audio from a YouTube video and convert it to MP3 audio you can throw on your player of choice. Not only can you rip the audio but you can specify what part of the audio you need. Only want the segment from 4:39 to 23:42? Not a problem with Dirpy.
In addition to simply ripping the audio, you can also edit the ID3 tags, so your download is already neatly tagged and ready to assimilate into your collection. If you find yourself wanting a copy of the video in addition to or independently of the MP3 file, you can download the video too. Dirpy is a free service and can be accessed both by visiting the main site or by using the Dirpy bookmarklet.