Gone - An ephemeral to-do list
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Streaks - The to-do list that helps you form good habits.
Workflowy - A better way to organize your mind.
Taskful - Deadlines, meet your match.