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

Matrix.org is a growing ecosystem for open, decentralised chat, VoIP and IoT communication.

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.

With Samepage, you can put all your project information on one page and collaborate with your team in real time. Samepage brings together an entire suite of project management tools, awesome communication features, and built-in integration with the apps you use. Everything you need to move projects forward, without leaving Samepage.

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.