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.
Anologue - Disposable online chat.
Chatib - Chatib is an online chat room where you can meet and chat with new people, make them friends, or even data them online.
EchoWaves - Opensource Social Group Chat written in Ruby On Rails
Chattusa - Chattusa is a social network website that provides a free chat service and lets you connect with people from the USA without any hassle.
JabbR - A chat application built with ASP.NET and SignalR.