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Mattermost is an open source, self-hosted Slack alternative.

Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.

HipChat is an instant messaging service designed for companies and teams. It has persistent chat rooms, one-to-one chat, searchable chat history, file sharing via drag & drop and clipboard, inline image previews, an API, chat over SMS, and more.

Self-hosted chat for small teams.

Gitter is a chat and collaboration tool for developers and users. It's a place for developers to talk.

eXo Platform is a cloud-ready enterprise portal and user experience platform-as-a-service (UXPaaS) for building and deploying transactional websites, managing web and social content and creating gadgets and dashboards. Built on open standards and open source technologies, eXo Platform has the features and extensibility of a UXP and the cloud architecture to build public and private clouds.

Zulip is a group chat app.

hack.chat is a minimal, distraction-free chat application.Channels are created and joined by going to https://hack.chat/?your-channel . There are no channel lists, so a secret channel name can be used for private discussions.

kune is a social network where you can communicate, collaborate and share between equals, using different web tools: documents, calendars, chats, wiki, messages, tasks, forums, barters.

dialog app is built and designed for corporate communication on all types of mobile devices - tablet, mobile phones etc.

Campfire is a simple web-based real-time group chat tool for business. It makes real-time communication with 2-60 people as simple as visiting a web page.

Open Source private chat server. Your own private Chat server in minutes, plus additional features. No credit card required.A fast, secure and stable solution based on Rails.