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.
Any.DO - The #1 task management app used by over 11 million people globally, Any.do is your free mobile and online task manager for Android, iPhone, Web and more.
Workflowy - A better way to organize your mind.
Microsoft To-Do - Task management tool
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.