Volker Weber notes the existence of a UNIX-based music server called daapd that streams files via the same DAAP protocol used by iTunes. The key advantage to using daapd is the ability to both stream and download tracks from locations outside of the local subnet.
As you may recall, iTunes 4.0.1 removed the ability to stream music from another machine running iTunes unless both machines are the same subnet. So if you want to listen to your own collection of music at home from a remote location, stick with iTunes 4.0.0 or use daapd in conjunction with cross-platform DAAP clients such as iSlurp or One2OhMyGod. The latter clients also allow you to download tracks, which isn’t supported even by iTunes 4.0.0.