avahi is a service auto-discovery that is installed by default on Debian and Ubuntu systems. On a Linux box avahi can be used for a number of things, including networked music sharing. Avahi runs mDNS plus DNS-SD: that is, multicast DNS plus DNS service discovery. Multicast DNS means that each equipped host stores its own DNS records. DNS-SD uses the same technology, but in addition to regular DNS information, hosts also publish service instance information: they announce what services they provide and how to contact those services.
See also Using Zeroconf on Linux on PracticallyNetworked.com.