Kontact - KOrganizer - KOrganizer is the calendar and scheduling component of Kontact, the integrated personal information...
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
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.
Teamweek - A project management software to help you get the best overview. Plan fast, change plans faster!
mailcow - An open source mailserver suite.