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Google Calendar - Spend less time managing your day & more time enjoying it
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
iCal - In Calendar on your Mac, subscribe to calendars published or shared by others.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.
Evolution - Apps/Evolution - GNOME Wiki!