Before Slack became popular, my old office used MacOS's chat app for messaging. As a kid who grew up on IRC, I was jealous of Slack bots and chat rooms.
This is a hacked together solution to get that working on Bonjour.
The interesting thing about Chat on OS X then is there was no sign up or chat server to be configured. You opened up the app, and you would see a list of usernames based on what people used to sign into their computer.
This worked because of Bonjour.