If you install daapd (a DAAP server), anyone running iTunes can connect to your shared library using a daap://your.server.org:nnnn URL. If you also install mDNSResponderPOSIX (a Rendezvous service), anyone on your local network will be able to see your shared library automatically in their iTunes source list.
If you point daapd to an often updated folder of MP3's, then use iTunes on the same machine, you'll have an iTunes library that is automatically updated (adding new files from that folder) each time you connect.