The best open source alternative to Slack is Mattermost, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Slack and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Slack are Element (Free Personal, Open Source), Zulip (Free Personal, Open Source), Rocket.Chat (Freemium, Open Source) and Matrix.org (Free, Open Source).
Librem Chat – end-to-end encrypted chat, VOIP, and video calling used by millions of people * Chat rooms * Share images, videos, and files * Make voice and video calls * Keep your conversations secret with end-to-end encryption * Free and open-source software
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.
Dirc is a peer-to-peer web application which uses WebRTC to set up connections between chat participants without requiring a back-end server. The static files can be self-hosted. Other features: * Identities are key pairs (NaCl).
MongooseIM is aimed at large, complex enterprise level projects, where real-time communication is critical for business success. It provides high availability, ease of deployment, development and reliability in production.
Communication is critical. Privacy is essential. Simple, just how you like it. Revolt is built on the same technologies as other popular chat applications, so there won’t be any need to re-learn on how to use it. Also, you can customize it to your heart’s content.