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Please read About This Article section in the end of this article for some explanation.

Telegram

A group chat application that is fast, user friendly, simple, popular and full-featured. It supports multi-accounts, multi-devices, group video call and screen sharing. It's available on web, mobile and desktop and can be accessed on all at the same time. See Telegram Features/FAQ. To start Telegram with your friends, download the application or simply use Telegram Web. We recommend Telegram FOSS version available on F-Droid for Android users. Visit Telegram.org. (GNU GPLv3+)

Signal

A group chat application that is popular in both computer security and FOSS community people. It's available as mobile and desktop apps. See Signal's Features and Documentation. To start Signal with your freinds, you can install the mobile app. Visit Signal.org. (GNU GPLv3/AGPLv3)

Matrix/Element

A new group chat technology, luxuriously featured, that connects to other messengers. It features group video call, screen sharing, file transfer and many more. It is available on web, mobile and desktop. See its Features and Documentation. You can start by visiting the web application Element.io, sign up, login and start group chat together with your friends. (Apache 2.0 License)

 

See the technology Matrix.org

 

Threema

A Swiss-based easy to use, full-featured, secure internet messenger. Formerly proprietary, it's been celebrated to become FOSS since 2020. It is fully available on mobile, desktop, and web. Group chats, video/voice calls, file transfer are a few of a lot of its features. No phone number or email required. See Features and Documentation. To start with Threema, you can start by installing its mobile version first. Visit Threema.ch. (GNU AGPLv3+)

Wire

A Swiss, European secure, good-looking and full-featured instant messenger. It was founded by ex-founders of Skype and then became FOSS since 2016. It is fully available on mobile, desktop and web. It features group chat, video/voice call and file transfer among others. See Feature List and Documentation.  Along with Jami, Wire is one among FSF's High Priority Project. To start, you can download the mobile application first. Visit Wire.com.

XMPP/Jabber

An older internet communication technology, full featured and is actually a world wide internet standard. It can be accessed via mobile, desktop, and web applications. We recommend you to start with Movim.eu (web), Gajim (desktop) or Conversations (mobile) so you can chat together with your friends. (Various free software licenses)

See the technology XMPP.org

Jitsi Meet 

A web video conferencing platform. It can be used instantly on web browser -- no signup required. See Features and Documentation. You can start by visiting meet.jit.si, create a meeting room, and invite your friends by link to start video/voice calling together. It is partly based on XMPP technology and is the video call part of Matrix/Element. (Apache License 2.0)

 

See the technology Jitsi.org

BigBlueButton

A web video conference platform for teachers & schools and is well-integrated with other Learning Management Systems (LMS). It is available on web only. See Features and Documentation. Account registration is required for speaker/moderator, not required for participants. You can start by signing up at BigBlueButton.org by hiting 'Try It Now' then create a room and invite your friends. (GNU LGPLv3)

DeltaChat

A unique, modern and secure email-based instant messenger. It's available on mobile and desktop. It features group chat, voice/video call via Jitsi Meet and file transfer. Delta allows users to communicate with other people who do not use Delta as long as they have email. To start, you and friends can install the app and chat together with your existing email accounts. Visit Delta.chat. (GNU GPLv3)

Nextcloud Talk

Talk, part of Nextcloud, is the modern web video conferencing platform integrated within the all-in-one enterprise-grade web server storage. In functionalities, it is similar to Google Meet. To use it, you need sign up to a Nextcloud server that offers Talk service and you can start here nextcloud.com/sign-up. (GNU AGPLv3)

Jami

Mattermost 

A full-featured teamwork chat for internal use of software developer corporations and organizations that present itself as a Slack and Teams alternative. It features project management, search, voice call & screen sharing and integrations. See list of Features and read the Documentation. You can start by signing up on Mattermost.com. (Apache License 2.0)

Rocket.Chat

A Slack-like general purpose teamwork chat. It is available on web, mobile and desktop. Being general purpose it is very similar to Mattermost but is made ready for education, government and healthcare aside from software development. It features group chat, voice/video call, and integrations. It is able to communicate outside to WhatsApp, Facebook Pages, Twitter, Email and others. Uniquely, it also is able to receive landline phone calls and supports federation to both ways Rocket.Chat and Matrix. See Documentation. You can start by signing up a service trial on Rocket.chat.

Zulip Chat

A Slack-like general purpose, full-featured teamwork chat. It is available on web, mobile and desktop. Zulip features group chat, unlimited search, file transfer, video/voice call like Mattermost and RocketChat, but is unique thanks to its messages organized by topics (like subjects in email) keeping busy workspace tidy. Email is required to signup. You can start with Zulip by joining their public group chat. Visit Zulip.com.

Revolt

Worth Mentioning 

FOSSCord 

A self-hostable Discord clone in short. In details, a Discord-compatible, full-featured communication platform featuring group chat, file transfer video/voice call that allows users to connect to both Discord server as well as Fosscord servers. It is still in early development stage with considerably limited number of users. Visit Fosscord.com.

Tox Chat: FOSS, fully decentralized, cross-platform, and full-featured with E2EE and PFS security measures but has a warning for still being not complete. Visit Tox.chat.

Briar: FOSS, fully decentralized, can even send messages without internet access, but text-only, still in alpha stage, and supports none other than Android. It's an ongoing project with big supporters behind like NLNet organization. Visit Briarproject.org.

Important Notes

You might have been familiar with other popular applications but wondering why they are not mentioned at the above. We explain briefly as the following:

WhatsApp / LINE / Google Hangouts & Meet / Zoom Meeting / Microsoft Teams / Facebook Messenger / Skype / Discord:

they are all nonfree software. It is also called proprietary software. It means end-users must agree to fully lose their rights to control (use-study-modify-share) the software code running in their device. Read more about Proprietary Software.

We hope they will turn into fully free software soon following success examples of formerly proprietary/nonfree messengers, Wire and Threema, and also formerly nonfree software desktop applications like Blender and LibreOffice. 

About This Article

FOSS means free/libre, open source software. Please refer to free software definition and open source definition.

Modern instant messenger means full-multimedia real-time communication and often includes group chat, file sharing, video and voice call features -- not just text.

Applications means this is about the software end-user runs in their computer should be FOSS. This does not require the server to be FOSS.

This article is a help for you together with your friends to choose FOSS communication apps.

This article is not a debate between various technical opinions. 

References

Free Software Definition 

Open Source Definition

Better Than Zoom by FSF 

Network Services are not Free or Nonfree ... by GNU Project & FSF

Remote Communication by FSF and LibrePlanet 

Skype Replacement by LibrePlanet

High Priority Projects by FSF

In Celebration of Becoming Open Source, ... by Threema

Instant Messengers Recommendation by PRISM-Break

Instant Messengers Recommendation by PrivacyTools.io

Instant Messengers Recommendation by PrivacyGuides

Better Than WhatsApp by FSF India

Secure messaging apps comparison 

Messengers Comparison by Threema

Messenger Comparison by Kuketz

Privacy is more than Crypto by FSF Europe