Open Source Messaging App
A private messenger is only as private as its weakest assumption, and for most apps that weak point is the metadata - even with encrypted contents, a central server still learns who talks to whom, how often, and from where. The open source apps here let you verify the encryption yourself and pick your threat model directly: some route through onion networks, some sync peer-to-peer with no account or phone number at all, so the social graph that betrays you stays off anyone's server.
Rocket.Chat
Open source team communications platform with messaging, voice, video, federation, and apps
Mattermost
Open source self-hosted collaboration platform with chat, workflow automation, voice, and screen sharing
Jitsi Meet
Open source video conferencing for browsers, mobile apps, self-hosted servers, and embedded meetings
Element
Matrix web and desktop client for end-to-end encrypted messaging and decentralized collaboration
SimpleX Chat
Private messenger with no user IDs, link or QR based connections, and double ratchet end to end encryption
Quiet
Encrypted p2p team chat with no servers, built on Tor and IPFS
Spacebar
Self-hosted Discord backend reimplementation with backward compatibility for existing bots, apps, and clients
Delta Chat
Decentralized desktop messenger with multi-profile, multi-device chat and chatmail relay support
Wire
End-to-end encrypted messenger for organizations, with a web client and server-side deployment