DevIM is Free Open Source Chat Console / Chat Client that can be operated or used for LAN Messenger or accross the Internet. It has very simple GUI design (looks like Command Prompt Windows). You can use it for Enterprise use or ordinary use.
Download DevIM Free
DevIM written in C++ Language. Before you install DevIM you may need to install Microsoft Windows Installer. It works well at Windows XP, 2000, and else (Not tested in Vista or Windows 7 yet). Download DevIM.
Official site: http://devim.sourceforge.net/