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.
Seald - Keep your conversations secure, everywhere.
ChatSecure - ChatSecure is a free and open source messaging app that features OTR encryption over XMPP.
Crypho - End-to-end-encrypted messaging and file sharing
Hermes - Pandora Client for OS X
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.