First let me state that this is not intended to do illegal music sharing across the internet; this setup is only intended to allow you to share your music with your family on the local network. I'm assuming you have setup a Linux server for Samba sharing or NFS. If you haven't, there are a number of how-to's and other resources on the web you can google.
You will need to download a few files to your Linux server first. Apple uses a protocol called daap to share music libraries. Rendezvous is also used to announce which services are available. You will need to download Apple's Rendezvous multicast DNS advertiser (free registration required) to make your Linux machine broadcast itself. You will then need to go to the daapd page and download all the appropriate files. I'm using Redhat 9 and everything compiled just fine on my computer. There is a sample configuration file included with the daapd server source code that you can edit and copy to /etc/daapd.conf.
Once everything is compiled, create a music folder on your Samba server and copy some music to it. Next you will need to run the multicast DNS advertiser from Apple with the following line as root user:
mDNSProxyResponderPosix 10.0.1.201 squeal "Music Jukebox" _daap._tcp. 3689 &10.0.1.201 is the IP address of your server and "Music Jukebox" is the name you will advertise to iTunes. Once that is running, you can edit the /etc/daapd.conf file to match your needs with directory, server name, and share name. Start the daapd daemon by typing this as root:
daapd &Once that is complete, you can open iTunes and it should pop up as a shared music library. This software is all in very early development so it doesn't do things like handle Album covers or updates to the library. It has some bugs still, but you can help the developers at the daapd page.

