Mattermost

Mattermost is an open source privacy conscious team collaboration platform similar to Slack. It can be self-hosted from your server at no cost or hosted in the cloud on a subscription and is available on all mobile and desktop systems. Written in Golang and React it features a clean and simple to use interface that allows your team to collaborate, communicate, and plan seamlessly.

To find out more about this team collaboration utility you can visit their website here.