Mattermost
Open Source Platform for Secure Workplace Messaging
Open Source Alternatives to Zulip
The team chat that helps teams stay productive and focused
Rocket.Chat
The communications platform that puts data protection first.
About Zulip
Zulip is an open source team collaboration tool that combines the concepts of email and chat to help teams stay productive and focused.
The Product
Zuplip offers server and client software that you can either host yourself or you can use their hosted service. Their threading model is designed to always see messages in context. It also offers a powerful enterprise features like SAML, LDAP, and more.