Mattermost

Mattermost wears its Slack inspiration openly: the same channel sidebar, the same composer, the same threaded replies in a right-hand panel. What sets it apart is the deployment model, since it is built to run on your own servers, and that single decision shapes the whole interface ethos toward control over flourish. The admin console is unusually deep, exposing permission schemes, channel and team management, and compliance toggles that consumer chat apps hide entirely. The result trades polish for governance: animations are sparse, the visual design is utilitarian, and onboarding assumes a technical operator stood the instance up. Markdown is first-class, slash commands and webhooks make it a hook-friendly automation surface, and board and playbook add-ons stretch it past chat. It reads as messaging for teams that need to own the box.