Mattermost
mattermost.com
Open-source, self-hostable team messaging platform — Slack-style channels, threads, and integrations under your own infrastructure control.
Why it is useful
Mattermost gives teams that need to keep chat data in-house (compliance, security, or cost reasons) a full-featured Slack alternative, including channels, threads, voice calls, and a large plugin/integration ecosystem. Being self-hosted also means no per-user SaaS pricing as the team scales.
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