iTunes had this back in the day. All the iTunes libraries on your network would show up in iTunes. It was awesome in college when entire dorms or apartment complexes would be on the same network.

iChat had a similar feature of showing people on your network. I messaged someone once to see if they also had an internet outage, because the local network seemed up. We got to chatting, and they told me they had been rocking out to my music library all year and invited me to a party. Great features.

Some people also made some software to pull the songs from these shared libraries. That was less legal, but the native iTunes sharing was awesome in the right environment.