LANcet Chat is free software for real-time message exchange over local network (LAN). With it you don't need to install chat server. All participants can connect any time and they will see each other automatically. Just run chat on every computer and start talking immediately.

Chat can freely maintain 10-20 users at the same time and even more. The only limitation for using it - you have to be inside single subnet, without software/hardware routers between users who want to participate.
Allows sending private messages. You can hold as much independent conversations as you want at the same time.
LANcet Chat can start automatically with Windows and stay in system tray (near the clock) waiting for messages whole day without disturbing you.
Popup windows and sounds will inform you when new message incoming or when new user appearing in your contact list.
You can switch off some boring interlocutors by "ignoring" their contacts.
Simple "Antiboss" system: press Esc and your chat will hide into system tray or will erase all the latest messages you got. Extremely useful when your angry boss is walking behind you and watching what are you doing on the job.