Mattermost

Mattermost is an open-core, self-hosted collaboration platform combining team chat with workflow automation, voice calling, screen sharing, and AI integration. The server is written in Go and React, runs as a single Linux binary backed by PostgreSQL, and ships extensibility through webhooks, slash commands, an API, and a marketplace of integrations.

Its licensing is layered: official compiled releases from Mattermost, Inc. are provided under the MIT license, while building the source code yourself is licensed under AGPLv3 (with some exceptions) or a separate commercial license, and a subset of admin tooling and configuration files is Apache-2.0 - reflecting Mattermost's open-core model rather than a single simple license.

It's commonly used for DevSecOps communication, incident response, and IT service-desk workflows by teams that want a self-hosted alternative to hosted chat platforms, with the option to deploy on-premises or use Mattermost's cloud offering.