Mattermost

Mattermost is a self-hosted team messaging platform built as an alternative to Slack. Channels, threads, direct messages, reactions, file sharing, and search work as you would expect, and the free Team Edition has no user limits and retains full message history.

The core appeal for self-hosters is data sovereignty. Your messages stay on your server. For teams in regulated industries or organisations with strict data residency requirements, that removes a category of compliance risk entirely. Mattermost is deployed by the US Air Force, NASA, and allied militaries precisely because it can run in air-gapped environments with no cloud dependency.

Integrations are where Mattermost earns its place in developer-heavy teams. Incoming webhooks, slash commands, and a bot framework mean CI/CD pipelines, monitoring alerts, and issue tracker notifications can all route through Mattermost rather than a paid Slack workspace. The plugin marketplace extends the platform further.

The honest trade-off versus the free tier: SSO, LDAP sync, compliance exports, and guest accounts require a paid Enterprise licence. For a small team that just wants persistent chat without a Slack subscription, the free edition is sufficient. For anything involving SAML login, Active Directory, or regulatory audit trails, budget for Enterprise.