I remember the old days when one person on a unix machine could chat with another person on a unix machine using the unix talk command, (piped over ssh for security)

I had many a conversation with my thesis supervisor this way, once when I was in France and he was in Japan.

PS no third party server involved, obviously. Just my box and the recipient box. I suppose one could do a man in the middle attack but we would always start the conversation with some pleasant banter anyway so it's unlikely that a third party masquerading as one of us could last long before detection.

<sarcasm>Too bad this technology no longer works, it was so simple and useful</sarcasm>