LAN Messenger - LAN Messenger is a decentralized UDP IM-style chat client which supports usernames...

Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.

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.

Keybase Chat - End-to-end encrypted chat built into Keybase

Bopup IM Suite - Organize secure messaging system and communicate instantly via self-hosted IM server.

Crypho - End-to-end-encrypted messaging and file sharing