Rocket.Chat
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Open Source Platform for Secure Workplace Messaging
Zulip
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About Mattermost
Mattermost is an open source platform for collaboration that is focused on software development teams and companies.
The Product
It has a self-hosted option and a cloud-hosted option. It offers secure, scalable, and private messaging, video, and voice calls. Mattermost also integrated with other tools like Jira, Jenkins, and GitLab.