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
Pithos - pithos is a daemon which provides an S3-compatible frontend for storing files in a Cassandra cluster.
Slack - A messaging app for teams who see through the Earth!
Keybase Chat - End-to-end encrypted chat built into Keybase