Ingamium is a plugin for the free chat client Adium. It allows you to receive messages and reply to them while playing a game without leaving fullscreen mode or quitting the game. You´ll simply see the messages on top of the game and in a small chat window you can view the last messages and reply to them.
Universal Binary (PPC/Intel)
Mac OS X 10.5 or later (10.6 recommended)
Adium 1.4 Beta (which can be downloaded from the Adium website).