DeskAlerts - Send alerts to any corporate device: PCs, phones, tablets, etc.
Rocket.Chat - Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.
Spectrum Chat - A newly open sourced self hosted Slack like alternative for building persistent chat communities
Slack - A messaging app for teams who see through the Earth!
Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Mattermost - Mattermost is an open source alternative to Slack.