LAN Messenger - LAN Messenger is a decentralized UDP IM-style chat client which supports usernames...
BeeBEEP - You can message and share files inside your intranet without a server.
MyChat - Secure instant messenger for any network
Squiggle - Squiggle is a free and open-source, peer-to-peer LAN messenger that is lightweight and feature-rich.
BORGChat - BORGChat is a LAN messaging software program.
IP Messenger - IP Messenger (Official Site)