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.

Kin Calendar - The simple and connected calendar

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

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

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

Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.