I have been using GNUMP3d for some time now, but a few things bother me.
For example, it uses a standalone web server to stream the music, which is useful in some cases, but requires you to set it up on a different port if you already run a web server. I know I could setup a mod_proxy or something similar, but I like it when things work right out of the box.

There are things out there like ampache, but nothing as light as GNUMP3d. That is why I coded a small streamer of my own using camping a Micro framework in ruby. There is not much code yet but it is already usable.

One of the advantages of using camping, is that it can work as a standalone server, but it can also easily be setup in a virtual host on apache, lighttpd or whatever.

Here is a screenshot of the latest (pre alpha) version:

As you can see it is very much like GNUMP3d in the way that it gives you access to playlists you can download and use in any music player.
I also added an embedded flash music player you can use to just listen in quickly.
MP3Camp supports m3u and xspf playlists and really needs some CSS job.

You can grab this early version on the svn (svn://svn.sig11.org/mp3camp).