The first instant messaging client I remember using was called BroadCast, in the early 1990s. It was a Mac chooser extension, meaning you launched it using the application you used to select a printer.
It ran over AppleTalk, was completely peer-to-peer, LAN-only (of course), and you didn’t have to add people to your buddy lists — if you had the BroadCast chooser extension, your name showed up in everybody else’s list and you could send and receive messages.
It also could message everybody on your network at once — handy when you want to warn people that the server was being rebooted, etc.
It was all so long ago that I cannot find a screenshot of BroadCast on the Internet except in one place — somebody seems to have made a Linux client for BroadCast.
