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.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
GTDNext - About Us.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Meistertask - MeisterTask is an intuitive task management and collaboration tool.