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.

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

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

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

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