ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
qTox - A Qt graphical user interface for Tox.
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.
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.
Otr.to - A secure peer-to-peer chat in your web browser.
uTox - µTox is the lightweight client with minimal dependencies; it not only looks pretty, it runs...