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

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.

Workflowy - A better way to organize your mind.

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

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

GTDNext - About Us.