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 Rocket.Chat, Matrix, Wire and Stoat Chat.
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.
monocles chat is a fork of blabber.im and Conversations with some changes, to improve usability.
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.
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.
Cowork is a free, gamified productivity app and virtual coworking space. It's built with task management features for students, colleagues, and freelancers. Users can create and customize shared coworking spaces for groups of people to work on their tasks together in and...
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.
Mostlymatter is a fork of Mattermost meant to remove the users and messages limits. Mattermost is an open source platform for secure collaboration across the entire software development lifecycle.
Pensil is a community building platform that brings together members, engaging discussion & live events for brands and creators.
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