AChat is a chat conversation application similar to the well-known IRC but especially programmed for local network environments. Furthermore, it's a completely free application.
The environment of the program is very similar to the well-known IRC, with a general chatroom and a list of connected groups at the right side. You can open private conversations in the group of your choice and also send massive messages. Also important is the possibility to send files with a complete transfer manager.
If you're looking for an efficient messenger for your office, AChat is one of the best alternatives you can find.