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.

Taskful - Deadlines, meet your match.

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

Calendarist - Time blocking companion for Google Calendar

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

Centered - Centered helps you get into a Flow State faster and stay there longer so you can do your best work.