Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Microsoft Teams - Microsoft Teams provides the enterprise-level security, compliance and management features you expect from Office 365, including broad support for compliance standards, and eDiscovery and legal hold for channels, chats, and files.
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Creativity 365 - Cross-device content creation suite