Centralized Messengers

Centralized messengers are those where every participant is on the same server or network of servers controlled by the same organization.

Signal is a mobile app developed by Signal Messenger LLC. The app provides instant messaging, as well as voice and video calling. All communications are E2EE unless you choose to send as SMS. Signal Messenger, LLC, is a software organization that was founded by Moxie Marlinspike and Brian Acton in 2018 to take over the role of the Open Whisper Systems project that Marlinspike founded in 2013. Its main focus is the development of the Signal app and the Signal Protocol. The organization is funded by the non-profit Signal Foundation, and all of its products are published as free and open-source software.

Threema is a free and open-source end-to-end encrypted instant messaging application for iOS and Android. The software is based on the privacy by design principles as it does not require a phone number or any other personally identifiable information. This helps anonymize the users to a degree. In addition to text messaging, users can make voice and video calls, send multimedia, locations, voice messages and files. A web app version, Threema Web, can be used on desktop devices. Threema is developed by the Swiss company Threema GmbH. The servers are located in Switzerland and the development is based in Pfäffikon SZ.

Wire is an encrypted communication and collaboration app created by Wire Swiss. It is available for iOS, Android, Windows, macOS, Linux, and web browsers such as Firefox. Wire offers a collaboration suite featuring messenger, voice calls, video calls, conference calls, file-sharing, and external collaboration –all protected by a secure end-to-end-encryption. Wire offers three solutions built on its security technology: Wire Pro –which offers Wire's collaboration feature for businesses, Wire Enterprise –includes Wire Pro capabilities with added features for large-scale or regulated organizations, and Wire Red –the on-demand crisis collaboration suite. They also offer Wire Personal, which is a secure messaging app for personal use.

Federated Messengers

Federated messengers use multiple, independent servers that are able to talk to each other (email is one example of a federated service). Federation allows system administrators to control their own server and still be a part of the larger communications network.

No phone number or email address is required to signup. Similar functionality like Element, but still in an earlier stage of development. Decentralized servers routed through Lokinet.

Element (formerly Riot and Vector) is a free and open-source software instant messaging client implementing the Matrix protocol. Element supports end-to-end encryption, groups and sharing of files between users. It is available as a web application, as desktop apps for all major operating systems and as a mobile app for Android and iOS.

Encrypted instant messenger with an integrated Ethereum wallet (cryptocurrency) that also includes support for DApps (decentralized apps / web apps in a curated store). Uses the Waku protocol (a fork of Whisper) for P2P communication. Only available for iOS and Android.

Peer to Peer (P2P) Messengers

Peer-to-Peer instant messengers connect directly to each other without requiring third-party servers. Clients (peers) usually find each other through the use of a distributed computing network. Examples of this include DHT (distributed hash table) (used with technologies like torrents and IPFS (InterPlanetary File System), for example). Another approach is proximity based networks, where a connection is established over WiFi or Bluetooth. Once a peer has found a route to its contact via any of these methods, a direct connection between them is made.

Jami (formerly GNU Ring, SFLphone) is a SIP-compatible distributed peer-to-peer softphone and SIP-based instant messenger for Linux, Microsoft Windows, OS X, iOS and Android. Jami was developed and maintained by the Canadian company Savoir-faire Linux, and with the help of a global community of users and contributors. Jami is free and open-source software released under the GNU General Public License. In November 2016, it became part of the GNU Project. By adopting distributed hash table technology (as used, for instance, within the BitTorrent network), Jami creates its own network over which it can distribute directory functions, authentication and encryption across all systems connected to it.

Briar is an open-source software communication technology, intended to provide secure and resilient peer-to-peer communications with no centralized servers and minimal reliance on external infrastructure. Connections are made through bluetooth, WiFi, or over the internet via Tor and all private communication is end-to-end encrypted. Relevant content is stored in encrypted form on participating devices. Long term plans for the project include blogging, crisis mapping and collaborative document editing. Briar's source code is published as free software and is distributed under the terms of the GPLv3 license.

Video/Voice Calling

Many of the above instant messengers also support VoIP. The software listed below are primarily voice/video focused.

Meetzi offers a wide range of features for collaborative work with extensive moderation capabilities. No registration required. Server in Germany. Runs directly in the browser. Data can be deleted immediately at any time. Including whiteboard, editor, Kanban board, file sharing and integrated external teaching materials. Currently, the service is offered only in German language. A multilingual website is in planning.

Jitsi is a collection of free and open-source multiplatform voice (VoIP), videoconferencing and instant messaging applications for the web platform, Windows, Linux, macOS, iOS and Android. The Jitsi project began with the Jitsi Desktop (previously known as SIP Communicator). With the growth of WebRTC, the project team focus shifted to the Jitsi Videobridge for allowing web-based multi-party video calling. Later the team added Jitsi Meet, a full video conferencing application that includes web, Android, and iOS clients. Jitsi also operates meet.jit.si, a version of Jitsi Meet hosted by Jitsi for free community use. Other projects include: Jigasi, lib-jitsi-meet, Jidesha, and Jitsi.

Linphone (contraction of Linux phone) is a free voice over IP softphone, SIP client and service. It may be used for audio and video direct calls and calls through any VoIP softswitch or IP-PBX. Also Linphone provides the possibility to exchange instant messages. It has a simple multilanguage interface based on GTK+ for GUI and can also be run as a console-mode application on Linux. The softphone is currently developed by Belledonne Communications in France. Linphone was initially developed for Linux but now supports many additional platforms including Microsoft Windows, Mac OS X, and mobile phones running Windows Phone, iOS or Android. It supports ZRTP for end-to-end encrypted voice and video communication.

Mumble is a voice over IP (VoIP) application primarily designed for use by gamers and is similar to programs such as TeamSpeak. Mumble uses a client–server architecture which allows users to talk to each other via the same server. It has a very simple administrative interface and features high sound quality and low latency. All communication is encrypted to ensure user privacy. Mumble is free and open-source software, is cross-platform, and is released under the terms of the new BSD license.

Team Chat Platforms

Mattermost is an open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams.

Rocket.chat is an self-hostable open source platform for team communication. It has optional federation and experimental E2EE.

Communication Services With Serious Security Issues
(THE USE IS STRONGLY DISCOURAGED)

Communication is not secure with the following services. Be aware of the danger of their use. With the below mentioned services, you have no control over which unauthorized or unwanted individuals or organizations may track your communications with certain or even full details. In addition, you give up the control over what happens to your private data and how they are used in favor of unknown groups and also against you. Furthermore, users expose themselves to dangers of cyber attacks and other criminal activities with the possibility of personal and financial damages. Please read the linked articles to learn more about the shortfalls.
The following list is not complete. It only shows a selection of the most popular services.

Comparison What Data An App Links To Your Account