iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Google Calendar - Spend less time managing your day & more time enjoying it
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.
CalenGoo - Features | 1.0.175 for Android™
mailcow - An open source mailserver suite.
Jorte - Schedule books must be practical and has a good design.