ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
Gajim - Full featured and easy to use Jabber client
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.
CoyIM - CoyIM is a safe and secure chat client.
ChatIW - Meet and Chat with new friends around the world.
Empathy - Apps/Empathy - GNOME Wiki!