Google Calendar - Spend less time managing your day & more time enjoying it

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.

Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.