Secure instant messaging system for team collaboration
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.
Softros LAN messenger - LAN messenger for an internal office chat for home or business users. It does not require a server and is very easy to install and use. It supports personal chats, group chats, message brodcasting, files transfer and remote desktop access.
Molly (Signal fork) - Molly is an independent Signal fork for Android
Bopup IM Suite - Organize secure messaging system and communicate instantly via self-hosted IM server.