IMix

iMix is a feature in the iTunes Music Store and iTunes software (by Apple Inc.) that allows users to publish playlists through the music store and share them with the public. The feature was introduced in version 4.5.

example: The Best of New Garage Rock (iTunes 4.5 required)

The above link returns an HTML page with some javascript to check for the iTunes player. If the user has iTunes, the javascript tells the browser to open an itms:// URL, which launches iTunes and opens the playlist.