I came across
Subsonic, a free software developed by Sindre Mehus, and I was instantly taken.
Well before Spotify made your songs available online, this sofware turned your home computer into a music streaming service. The application itself is Java based and needs you to run an instance of Tomcat, but you can simply install it through the provided installer.
There are now five applications for various devices out, but I would want to mention the Android app, as it's developed by Sindre Mehus himself. The Subsonic Android app supports streaming, downloading, playlists, album art and searching. For improved performance, music you have listened to is cached on the phone. An offline mode is also available for when you are outside 3G/Wi-Fi coverage.
All in all it's a great app when you happen to like music and you have got your entire music collection on one computer at home... Get it from the Android market and start listening to your favourites today...