Mattermost

Mattermost started as an internal tool at a gaming company before being open-sourced in 2015, and that origin shaped its character: it was built by engineers, for engineering and operations teams, with compliance and control as first-class concerns. The server is written in Go and ships as a single binary or Docker image. Channels, direct messages, group messaging, and threaded replies cover the collaboration basics, but the differentiation is in the adjacent features — built-in Playbooks guide teams through incident response runbooks, Boards provide lightweight kanban project tracking, and the slash-command framework lets DevOps teams wire in deployment triggers, on-call rotations, and monitoring alerts directly inside channels.

The security posture is the headline. Mattermost supports SAML 2.0, LDAP/AD sync, MFA, and full audit logging with e-discovery export. Because the entire stack runs on infrastructure you control, there is no data leaving your network — a requirement for defense contractors, financial services firms, and government agencies that cannot use cloud-hosted chat.

The MIT-licensed Team Edition is free to self-host with no user cap. Enterprise Edition, which adds advanced LDAP, SCIM provisioning, high-availability clustering, and the more sophisticated compliance controls, is licensed commercially. The hosted SaaS offering starts at $10 per user per month. The honest trade-off: the mobile and desktop apps, while functional, have historically trailed Slack in polish, and the administrative surface area requires someone comfortable managing a server process.