Thanks to a discussion on the Nekochan.net I have found that there is something called IP Messenger. It’s a program for sending of short messages (and files). It requires no server and it is probably meant for internal networks.
It is interesting that there are clients/clones for many platforms (including Gtk+ annd Qt ports) and it is quite small and fast. I don’t plan to use it often but it looks like a nice tool.
P.S. The screenshot shows the Motif clone (just created by “geo” from the Nekochan.net) which work also on SGI computers (it canbe compiled for other systems, including Linux ones, too).