I used to really like ICQ's floating usernames and email-like modal. A message came in and a little icon appear beside the username (which I used to float in the unused part of my menu bar) along with an audible notification "uh oh!" You double click it, a small chat window opened up with your conversation history and a place to type. You then typed and send your message and the window disappeared until they replied. It was great because you could see who the messages were from without having to use any screen real estate.

Then things like MSN Messenger got popular. Protocols got locked down and screen real estate got consumed. For some reason everyone wanted to use Google Talk (which was horrible but at least spoke XMPP so folks on Pidgin could still chat with their friends). BB messenger taught a few tricks to Apple. Then facebook messenger happened, whatsapp, wechat, etc.

In the background, there were programs like teamspeak and mumble that were trying to do voice chat for gaming reasons. In a way, these were the precursors to Discord who now, for some reason, dominates what people consider a good chat UI. I personally think it's a bloated mess. In my opinion, ICQ had the best chat UI. Because it stayed out of the way.

It's too bad it turned out to be spyware! Hah