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.
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.
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.
Workflowy - A better way to organize your mind.
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring