More recently, I used Songbird (which went through a few name changes). It was a web browser that tried to include iTunes functionality. It really sucked - imagine trying to take a normal web browser and make it twice or thrice or more slower - but the idea was nice, you could go to music blogs, and click the play button and listen to all the songs they linked to, and click the download button if you wanted to keep the file.
I am trying to get Nuclear to work, just to see what it's about, but I literally can't get any audio out of the program. That makes it slightly useless.
I would love a program that collated legally available music and provide a music player interface (preferably VLC or WinAmp style and speediness). There is so much music out there that you would never have to listen to the same song twice. Digging through SoundCloud and BandCamp and Archive.org would provide endless music. Archive.org alone, if you wanted to avoid legal issues (even though lots of music is available free streaming at the other two websites I mentioned).
Perhaps I'll have to write this program myself. I already have too many projects as it is, though.