Mattermost

Mattermost is an open-source, self-hostable collaboration platform built for organizations that cannot put their conversations in someone else's cloud. Started as a game studio's internal chat tool and open-sourced in 2015, it grew into the leading self-hosted alternative to Slack and Microsoft Teams, combining channel messaging, voice and screen sharing, incident-response Playbooks, and integrated AI agents. Its customers skew toward defense, government, financial services, and technology organizations - from the U.S. Air Force to Uber - that need data sovereignty, air-gapped deployment, and government-grade security.