mDNS Insight listens to multicast DNS (mDNS, also known as Bonjour or Zeroconf) on the local network, and lists each device it finds along with the DNS-SD services it advertises: printers, speakers, smart home hubs, file shares and anything else that announces itself. It decodes what those services say about themselves, so a printer's TXT records read as a model name and a list of paper sizes rather than a wall of key=value pairs.

Open it from your desktop's application menu to watch the network live in a web UI that opens in your browser. Or run it from a terminal:

 mdns-insight                        # discover for a while, then report
 mdns-insight -output-format=json    # the same report, for scripts
 mdns-insight -webui                 # the live web UI, no browser
 mdns-insight -open                  # the live web UI, in your browser

The web UI listens on this computer only, at a port the system picks. -webui=ADDR serves it elsewhere, for example to reach it from another machine on a network you trust.