Purp - A smart to-do list and goal tracker app
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Things - Things is an easy to use task manager.
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.
Nirow - Habit tracking: Automated, Siri-ed, Calendar-ed, and More!
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.