Open alternatives4
Open-source apps that replace Slack and Microsoft Teams

Rocket.Chat is a team messaging platform for real-time chat, voice calls and federation between organizations. It is aimed at workplaces and organizations that want to host their own chat with strong data protection.

Mattermost is a team chat platform that you install on your own server, so your company's messages stay under its control. It is aimed at workplaces and IT teams that want chat, workflow automation and voice calls in one place.

Zulip is a team chat app where conversations are organized by topic, so it combines features of email and chat. It is for workplaces and open-source projects, and can be used on the web or self-hosted on your own server.

Element is a chat app for private messages and group chats over the Matrix network, used in a web browser or as a desktop app. It is for people and teams who want chat they can host and control, with phone apps offered as separate Element X projects.
What's missing
What people still miss here, and wish existed. Upvote what matters to you.
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About this page
Replaces fully means it does the main job of these apps; Replaces partly means it covers some of it.
Licenses: every project here uses an open-source license (what each one means). Open core means some parts are paid or under another license.
GitHub stars are shown for reference. They measure popularity, not whether it replaces anything.
Verified means r/TrueOpenSource mods checked it: open-source license, maintained, and it really does the job.
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