Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Gone - An ephemeral to-do list
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Streaks - The to-do list that helps you form good habits.
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.