Subsonic is a free web based music streamer. with it installed on your computer you can access your music anywhere that you have access to the Internet and on any device that has a Flash enabled browser (in addition the iPhone, Android, Windows, Linux, and Mac have specific applications for use with Subsonic).
Subsonic works with all stream able audio formats. Additionally, it can use on-the-fly conversion to convert any non stream able format and stream it to the web player.
As well as streaming the music to your browser it can also operate in Jukebox mode, allowing you to control what your host computer is playing.
Additionally, you can use it to upload any file to your computer.
These last two features work particularly well with the robust user account system. It is easy to set up any number of accounts for yourself or your friends and even includes a IM IRC style chatting system allowing you to collaboratively set up playlists or build a collection of music.