Download Emacs definitions: imimacs.el (requires GNU Emacs >= 22.0.90)

A screenshot is here.

This gives you a very simple instant messaging system for Emacs. You can send messages to and receive messages from any peer that has imimacs installed. To install it, copy imimacs.el to your load-path and add to your .emacs: 
        (require 'imimacs)
	; interface that the server listens on:
        (setq imimacs-listen-interface "eth0")
	; predefined peers available via M-n and M-p in minibuffer:
        (setq imimacs-peers '("192.168.123.12" "my.peer.com"))
        ; arbitrary (nick-)name, prepended to outgoing messages if non-nil
        (setq imimacs-user-name "my_nick")
        (imimacs-init)
        (global-set-key [f5] 'imimacs-send)
Restart Emacs and press F5 to send something to a peer; M-p and M-n cycle through addresses predefined via `imimacs-peers'. C-0 F5 sends a message to the peer you got the most recent message from.

Adjust `imimacs-receive-face' and `imimacs-send-face' to change text properties of received and sent messages. For example, try: 
        (setq imimacs-receive-face '((:background "LightCyan2")))

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