The Tcl IRCd is a small IRC server written in Tcl. It is very simple to modify. It is not a full implementation of the IRC protocol, but it's enough to create channels, talk in public or private, change topic, nick, and other basic operations. No configuration is required. The server is designed so that it can reload an updated version of itself at runtime without closing client connections.
AIMSpy is an application that parses and displays content from instant messaging systems using the local network. It is multi-threaded, uses BPF devices for network access, and includes fairly thorough support for the AIM chat protocol and rudimentary support for the Rendezvous (iChat) protocol.