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

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

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

Workflowy - A better way to organize your mind.

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.

Remember The Milk - Remember The Milk is a task and time management application for mobile devices.