Subsonic is a free Java-based media streaming tool that allows you to stream music over your network, or the internet.
The system works best when streaming MP3 files, but support for transcoding plug-ins allows it to convert and stream most other audio formats, too (WMA, OGG, FLAC, AAC and many more).
Subsonic includes plenty of configuration options to help ensure that it can deliver the service you need. If you're hoping to stream across a relatively slow wireless network, for instance, you're able to have your music resampled to reduce its bitrate, minimising the chance of any dropouts.
And the program offers many of the fun extras that you'd expect from a music manager. So you can use it to locate cover art, find lyrics, read reviews, download and listen to Podcasts, and more.
While the program is free, if you make a donation to the project then you'll get even more features, including video streaming, the ability to stream music to Android, iPhone and Windows Phone 7 devices, and a personal server address (yourname.subsonic.org). Check the author's site for more information.