Keybase Chat - End-to-end encrypted chat built into Keybase
Slack - A messaging app for teams who see through the Earth!
Pithos - pithos is a daemon which provides an S3-compatible frontend for storing files in a Cassandra cluster.
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.
Pandora - Pandora Radio is a free (ad-supported) internet radio founded in 2000.
Mattermost - Mattermost is an open source alternative to Slack.