RocketChat - The ultimate open source web chat platform

•    Coffeescript

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Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework. It is a great solution for communities and companies wanting to privately host their own chat service or for developers looking forward to build and evolve their own chat platforms. Its features include File sharing, Video conferencing, Screen sharing, Live chat, Link preview and lot more.

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Gitter - Community for Software Developers

•    Javascript

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Gitter is designed to make community messaging, collaboration and discovery as smooth and simple as possible. You can easily create, organise and grow your communities, inviting others to join just in one click. Gitter is an instant messaging and chat room system for developers and users of GitHub repositories. Gitter is similar to IRC and Slack. Unlike IRC, and like Slack, it automatically logs all messages in the cloud.

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Mattermost - Self-hosted Team Communication Service

•    Go

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Mattermost is self-hosted team communication service bringing messaging and file sharing into one place, accessible across PCs and phones, with archiving and search.

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Zulip - Group Chat

•    Python

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Zulip is a powerful, open source group chat application. It supports both private messaging and group chats via conversation streams. Zulip also supports fast search, drag-and-drop file uploads, image previews, group private messages, audible notifications, missed-message emails, desktop apps, and much more.

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Matrix - Decentralized group communication

•    Python

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Matrix is an open standard for decentralised communication, providing simple HTTP APIs and open source reference implementations for securely distributing and persisting JSON over an open federation of servers. It can be used for Decentralised Group Chat, WebRTC signalling, Internet of Things, VOIP etc.

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Lets-Chat - Self-hosted chat for small teams

•    Javascript

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Let's Chat is a persistent messaging application that runs on Node.js and MongoDB. It's designed to be easily deployable and fits well with small, intimate teams. It supports XMPP Multi-user chat (MUC), Multiple rooms, File uploads and lot more.

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braid - Braid is a team-chat app with a novel UI that leads to better conversations.

•    Clojure

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Braid is an experimental team chat application with a novel UI: instead of chatrooms, it's based on many short conversations. You can think of Braid as an email/mailing-list/web-forum/chatroom hybrid, but without the constant interruptions and FOMO of Slack. It's particularly good for remote-first companies, developer teams and online communities. Braid is also open-source, so you can hack on it and host your own version. You can read more about Braid and then give Braid a try.

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Friends - P2P chat powered by the web

•    Javascript

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Friends is like Slack except P2P, offline friendly and open source. Data is transmitted over secure channels and messages are authenticated. It is made by a group of open source developers who are motivated to make a public chat platform that is easy to use but isn't centrally controlled, and where data is owned by its users.

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