Mattermost

Mattermost helps you and your teams increase productivity by delivering real-time chat in team-based communication channels. It provides the ability to securely share information, files and content from dozens of your commonly used business applications instantly.

Using a browser or an app on your mobile: real-time communication is now available without compromising privacy or control of your data.

Mattermost is the number one open source Slack alternative.and can be run on a self-hosted platform under your control.

Mattermost connects easily to 3rd party applications and systems like Gitlab, Jira Jenkins, Nagios, Zabbix, Kopano and a lot more!