Adium

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Bitmessage

Jitsi is an open-source project and software toolset that allows you to easily build and deploy secure video conferencing solutions. At the heart of Jitsi are Jitsi Videobridge and Jitsi Meet, which let you have conferences on the internet, while other projects in the community enable other features such as audio, dial-in, recording, and simulcasting.  Jitsi supports popular protocols such as SIP, XMPP/Jabber, ICQ/AIM, Windows Live, Yahoo!, Google Hangouts, OTR, ZRTP and more.

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Pidgin is a chat application which lets you log into accounts on multiple chat networks simultaneously. Pidgin is developed and released fully free and open-source and provides a plugin architecture that allow anyone to build extensions and connections to new networks and third-party services.

Element is an All-in-one secure chat app for teams, friends and organisations that allow you to own your conversations. With Element you can keep conversations in your control, safe from data-mining and ads. Element utilize the open and global Matrix network for message relay, protected by proven end-to-end encryption.

Matrix.org is an open network for secure, decentralized communication. It gives you a simple HTTP APIs and SDKs (iOS, Android, Web) to create chatrooms, direct chats and chat bots, complete with end-to-end encryption, file transfer, synchronised conversation history, formatted messages, read receipts and more. All conversations on Matrix are replicated over all the servers participating in them, meaning there are no single point of control or failure.

Signal is a cross-platform encrypted messaging application that has a strong focus on privacy. The application is developed by Signal Messenger LLC and provides users with features like one-to-one and group messages, which can include files, voice notes, images and videos. The software is famous for being used by the whistleblower and privacy advocate Edward Snowden who once said "I use Signal every day, it is the most scalable encryption tool we have".