Riot allows teams to communicate across a wide range of collaboration apps. If some team members use Riot while others use IRC, Slack or Gitter, Riot will allow these team members to seamlessly work together. Riot offers the richest network of communication bridges. Riot lets you run your own server, and provides users and teams with the most advanced crypto ratchet technology available today for a decentralized secure Internet. Riot is entirely open source: all the code is published on GitHub (Apache License) for anyone to see and extend. This means teams can customize or contribute to the code and everyone can benefit from the speed of community innovation. Riot is built on top of Matrix.

Mattermost is a self-hosted solution for bringing all your team communication to one place, and making it searchable and accessible anywhere

Powerful slack-like open source group chat