Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
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.
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.
Things - Things is an easy to use task manager.