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.

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.

Jorte - Schedule books must be practical and has a good design.

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

Business Calendar - Powerful and easy to use calendar app for Android including weather, widgets and tasks.