Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Doit.im - Doit.
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.
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.
Astrid - Tasks is free and open source task management software based on the same code as Astrid! The complete source code is available at https://github.