Open source alternatives to Slack

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mattermostv11.11.4mattermost/mattermost

Mattermost is an open source platform for secure collaboration across the entire software development lifecycle..

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Rocket.Chat9.0.0RocketChat/Rocket.Chat

Interoperable by design. Governed by you. Open-source, secure platform for messaging, voice, video and governed AI, built for defense and government.

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zulip13.2zulip/zulip

Zulip server and web application. Open-source team chat that helps teams stay productive and focused.

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synapsev1.165.0element-hq/synapse

Synapse: Matrix homeserver written in Python/Twisted + Rust