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.
Streaks - The to-do list that helps you form good habits.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Taskful - Deadlines, meet your match.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
ToDoList - Windows desktop application to stay on top of your daily tasks.