iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
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.
Google Calendar - Spend less time managing your day & more time enjoying it
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Lightning Calendar - Organize your life — it’s about time!