Mattermost

Mattermost is an open-source, self-hosted messaging and collaboration platform built for teams that need to keep their communication on their own infrastructure. It is widely positioned as a privacy-focused alternative to Slack, with a Go backend and a React-based web client, and it requires a PostgreSQL (or MySQL) database for storage.

Key capabilities include organized public and private channels, direct and group messaging, threaded conversations, file sharing and search, slash commands, webhooks, and a rich integration ecosystem including bots and plugins. It also offers desktop and mobile clients, single sign-on options, and developer-oriented features such as code formatting and ChatOps workflows.

Mattermost is typically used by engineering teams, government bodies, and security-conscious organizations that need full control over their chat data and deployment. A small single-team instance runs comfortably alongside its database on a modest server, while larger deployments scale out with dedicated database and storage backends.