Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.
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.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.