The best open source alternative to Slack is Element. It's not free, so if you're looking for a free alternative, you could try Element or Zulip. If that doesn't suit you, our users have ranked more than 100 alternatives to Slack and loads of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Slack are Matrix, Wire, Stoat Chat and Threema.
Zero Knowledge, end-to-end encrypted collaboration tool for teams and businesses. Reviewable source code. Python wrapper for bots & integrations. Encryption white paper. Native offline mode. Faster file-sharing. Built for enterprise.
Valour is an open-source, privacy focused alternative to platforms like Discord or Slack, developed by a small team of volunteers. The app has been in development for over three years and is currently in it's alpha stages of testing.
Conduit is a simple, fast and reliable chat server powered by Matrix. Conduit is an alternative to Synapse and tries to be lightweight and easy to install, but it is still in development.
Awesome hosting for Matrix chat and integrations - built on the Matrix open ecosystem for secure, decentralised communication, by the creators of Matrix.
hack.chat is a minimal, distraction-free chat application. Channels are created and joined by going to https://hack.chat/?your-channel. There are no channel lists, so a secret channel name can be used for private discussions.
Source code : https://github.com/hack-chat/main.
PrivMX Fusion is safe and easy collaboration software for all kinds of teams, with virtual workspace protected by end-to-end encryption. Each team uses desktop/laptop client software and its own (zero-knowledge) PrivMX Team Server.
monocles chat is a fork of blabber.im and Conversations with some changes, to improve usability.
Kahla is a free and open-source, cross-platform business messaging tool that supports end-to-end encrypted message communication and automatic destruction of messages.
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