qOrganizer!

Microsoft Outlook, part of the Microsoft Office Suite (not to be confused with Microsoft Outlook Express), is a personal information manager that lets you manage your e-mail, calendar, contacts, and tasks.

EssentialPIM lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

Keep track of your busy schedule and share it with others using Calendar. Create separate calendars — one for home, another for school, a third for work, and so on. See all your calendars in a single window or choose to see only the calendars you want. Calendar lets you create invitations using information from Contacts, update your guest list, keep track of responses, and receive the latest status information. When you or another Mac user receives a Calendar invitation via Mail, it’s automatically added. Calendar also works with iCloud, Yahoo!, and Google. And when you use iCloud, every Calendar edit you make on your Mac automatically appears on your iPad, iPhone, and iPod touch. So no dinner dates are missed, no meeting goes unattended, and no anniversary is forgotten. »

Rainlendar is a feature rich calendar application that is easy to use and doesn't take much space on your desktop. The application is platform independent so you can run it on all major operating systems: Windows, Linux and Mac OS X. The appearance can be customized with skins and you can even mix different skins together. It is possible to use Rainlendar with other calendar applications too. Rainlendar uses the standard iCalendar format to store its events and tasks so you can easily transfer them between applications. You can also subscribe to online calendars including the Google Calendar and edit your Outlook appointments directly in Rainlendar*. »

Fantastical is a flexible calendar with natural language event creation, a beautiful calendar and event list, support for CalDAV (including Google Calendar delegates) automatic alarms, and instant searching.

MyLifeOrganized (MLO) is a personal task outliner and to-do list manager which will help you to organize your goals, projects and tasks into a tree.

An offline calendar without any other calendar integration. You can easily create recurring events and setup reminders, it can also display week numbers.

BORG is a combination calendar and task tracking system. The calendar supports all sorts of appointments and also a simple todo list. The calendar functionality is similar to that of other PIMs, such as Microsoft Outlook, Mozilla Calendar, Palm Desktop, Yahoo Calendar, etc... The Task Tracker goes way beyond todo's and might better be called a project tracking system.

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Share Calendars on a LAN and sync with Google Calendar.

This program helps you organizing your dates and tasks and reminds you of upcoming events.