Open fire is launched by Ignite Realtime, its a open source. It provide instant chat service which can be installed on windows server and even on linux.
If you are looking for free chat server, openfire is real a good choice, it can be integrated with active directory, users can be created under different groups. You get many of other plugins which can be installed according to any organization requirements.
Download Link: http://www.igniterealtime.org/downloads/index.jsp#openfire