Subsonic is a free, web-based media streamer, providing ubiquitous access to your music. Use it to share your music with friends, or to listen to your own music while at work. You can stream to multiple players simultaneously, for instance to one player in your kitchen and another in your living room.
Alternatively, you could consider streaming MPD with Icecast.
Icecast is a program for streaming audio such as music across a network. Different types of clients connect to the IceCast server, either to provide a "mount point", control the server, or listen to the audio being cast.
MPD is a program for playing music via a daemon process instead of using a client. It also incorporates a music database for quick access, playlists, and a variety of front-end options.