Mattermost

Mattermost is an open-source alternative to Slack and Microsoft Teams. They offer a high trust, Slack-compatible messaging, and collaboration platform for enterprises that value security, flexibility, and innovation.

Mattermost is the world’s largest open source project for digital workplace technology. Collaboratively developed by over 500 contributors, the platform is available in 16 languages with a rich and broad ecosystem of integrations and enhancements.