Shtuff Chat - Free Group / Team chat with persistent history.
Element.io - Secure messaging app with strong end-to-end encryption, advanced group chat privacy settings, secure video calls for teams, encrypted communication using Matrix open network. Riot.im is now Element.
JabbR - A chat application built with ASP.NET and SignalR.
Chatib - Chatib is an online chat room where you can meet and chat with new people, make them friends, or even data them online.
Smarsh Instant Messenger - smarshIM Mobile is a free application download that requires a paid account on the smarshIM service...
Kandan - Open Source private chat server. Your own private Chat server in minutes, plus additional features.