Keybase Chat - End-to-end encrypted chat built into Keybase
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.
Hermes - Pandora Client for OS X
Brie.fi/ng - Start secure group video chat
dPhone - The most secure calling app. Open Source and Free.
Slack - A messaging app for teams who see through the Earth!