LAN Messenger - LAN Messenger is a decentralized UDP IM-style chat client which supports usernames...
RealPopup.it - A network messaging application that's feature-rich yet easy-to-use and won't cost you a fortune, RealPopup LAN chat offers fast, stable messaging across your local network. No application server required, just install on your PC and start messaging!
Bopup IM Suite - Organize secure messaging system and communicate instantly via self-hosted IM server.
Output Messenger - Secure instant messenger app for office provides seamless instant messaging solutions for all office, corporate and enterprise business communication needs.
MyChat - Secure instant messenger for any network
BeeBEEP - You can message and share files inside your intranet without a server.