Mattermost

Mattermost is an open source team chat tool that you can host yourself. It covers channels, threads, direct messages, search and workflow automation in one place.

Teams talk in channels and threads, send direct messages, and search through the full history. Admins can set permissions and retention per channel. Workflow automation and project tasks sit alongside the chat, and an integration framework connects it to other tools. Because you control the servers, you decide where the data lives.

• Team messaging with channels, threads and search
• Searchable history with per-channel permissions and retention
• Workflow automation and project tasks
• Runs self-hosted, in a private cloud or air-gapped
• Available on web, mobile, and inside Microsoft Teams

Mattermost suits engineering and IT teams who want chat under their own control, and organizations with strict data rules. The company's site focuses on government, defense and critical infrastructure, but the core product is team messaging for any group that wants to host its own chat.

The project is open source, so you can self-host the core product without a license fee. The company also sells Enterprise tiers, including an Enterprise Advanced edition with extra security features. Running your own servers means you also manage updates and backups.