If you only need a small chat program without unecessary stuffs, Lan-Talk is for you ! It is based on a Server/Client architecture, so you will need a server or whatever which can handle the server side (fixed IP, reachable by all client, ...)
I'm making it in C++/Qt4.4 so it will run on all Qt4 supported platforms ( Linux / OSX / Windows/ WinCE / ... )
