FluffyChat - Open. Nonprofit. Cute ?
LAN Messenger - LAN Messenger is a decentralized UDP IM-style chat client which supports usernames...
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
MyChat - Secure instant messenger for any network
Molly (Signal fork) - Molly is an independent Signal fork for Android
BORGChat - BORGChat is a LAN messaging software program.