In the new iTunes 4, Apple enabled the ability to stream over a network, but you can also use this new feature to stream over the Internet with the help of a new freeware application called ServerStore. By using ServerStore and its database of published connections (all obtained by user submissions) you can browse other people’s shared libraries. You can also publish your own connection in the database of published collections.
Remember that sharing music in this way requires a great deal of bandwidth and connection speed / network congestion will affect the quality of the ‘stream.’ Also, this feature is for personal use only.
To setup your copy of iTunes to share libraries and playlists, open the iTunes preferences and navigate to the “Sharing” tab. Select “Share my music.” Then select whether to share all of your music or just specific playlists. Then simply enter the name of your share (the name people will see when logging into your library) and require a password or not for your share to be accessed.
More information and download link here.