Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Rocket.Chat - Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.
Ripcord - A desktop chat client for Discord and Slack
Slack - A messaging app for teams who see through the Earth!
Ziggs - Smoothly Share Content Between Devices!
Weavy - The complete white-label framework for in-app team messaging and collaboration.