Akon!, is a simple to use, audio conferencing tool, that supports sending messages to users on your local area network.
Messages, can be voice based, as in a one-one audio chat or in a group, where everyone gets to listen what you said.
You get full control as to whom you speak to & whom do you listen at a particular time.
Akon! also supports, sending short single line messages to other users on the LAN, which pop up as system notifications.
Other notification, such as when a user joins in or goes out, or just changes his / her status are also supported.
And, it is written in Java, so it is almost platform independent.
Exceptions are, when you are using a non-GUI based system.