Zephyr
From KGB Wiki
Zephyr is an ancient art of internet communication. Messages, called zephyrgrams, are authenticated via Kerberos and addressed to a tuple of a class, an instance, and a recipient; this enables both targetted and broadcast styles of communication. Zephyr is slightly more heavyweight in feel than (though similar in style to) most popular instant-message programs, but it remains far "lighter" than email or usenet.