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.
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
Chatib - Chatib is an online chat room where you can meet and chat with new people, make them friends, or even data them online.
Keybase Chat - End-to-end encrypted chat built into Keybase
Kandan - Open Source private chat server. Your own private Chat server in minutes, plus additional features.
Hermes - Pandora Client for OS X