Google-Searchable and community focused Slack alternative. Linen is built for community support. Linen is designed to be familiar to existing communities while giving all the benefits of a forum.
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Mattermost
Mattermost is an open source Slack alternative. It's written in Golang and React and runs as a single Linux binary with MySQL or PostgreSQL. Use the features you like (file sharing, real-time group chat and webhooks—to name a few) and access the source code.
Spectrum
Spectrum is a community platform for the future. By building on Spectrum, communities become easily discoverable through search, curation, and even other community members.
Markdoc
Markdoc is a powerful, flexible, Markdown-based authoring framework. From personal blogs to massive documentation sites, Markdoc is a content authoring system that grows with you.