Songza

This site is great - type in the song you're looking for, and it serves up multiple versions, including rare recordings, which you can listen to immediately - it starts streaming as soon as you hit Play.

How do they do it? I have no idea.

You can build playlists too.

If you can't think of anything to search for, you can peruse the Top list, which, amazingly enough, featured Snow
Patrol and Kings of Leon songs. So the Brits must use it a lot. The Featured list is sponsored by Songza, and has both new and slightly esoteric choices as well as standbys.

Rate the songs, send them to friends, or create a playlist just by clicking on a song. Super simple site to use, dynamic functionality.