Telnetchatd is a chatserver which uses the telnetprotocol. Users do not need to install a client, as every desktop-OS comes with a preinstalled telnetclient. Programmed in pure C, the server needs very little resources and runs on any unix-like system.
YaCS - A window based chatserver with the ability to log or kick users. You can connect to the server by using PuTTY or Microsoft Telnet, because the chat supports VT100 and the telnet protocol.