Ubuntu offers several methods to share your music library over the network:
Easiest method as of 10.10 is to use the default Rhythmbox Music Player:
1. Edit > Plugins to bring up the Configure Plugins Dialog:
Enable DAAP Music Sharing plugin to share over the network
2. DAAP Music Sharing plugin. There is a Configure option that brings up a dialog:
(Click image to Enlarge)
3. Profit.
Other Options
Banshee music player is / will also be developing a DAAP Music Sharing plugin. It relies on an external application called Tangerine to provide media server capability.
Another interesting option is the non-gui media server option mt-daapd aka Firefly Media Server, available in the Synaptic Package Manager. mt-daapd is being replaced with forked-daapd, but that is not yet available to Ubuntu without going upstream to Debian. I have written a little piece providing more details of installing Firefly Media Server (mt-daapd) including some screenshots.