From Simple Products:
GameChat is an open-source IRC-like chat client. GameChat is built for gamers, who enjoy playing with people and who is looking for an easy way to interact with other gamers. GameChat is what we call game ready which means GameChat is compatible with any game that can run in windowed mode. GameChat does not affect any game performance, therefore no lag is caused by GameChat. *Note: someone who you will be chatting with (or yourself) needs to be running the supplied Dedicated Server and give out the IP to everyone else, which they will then use to connect to the server. This program will not connect to MSN, Yahoo!, AIM, and IRC chatrooms.