The Habit List app by Scott Dunlap & Gerard Gualberto is very promising.
The app allows you to create a list of habits, which can be set to repeat daily (or by a variety of choices—for example, every Monday Wednesday and Friday).
As you complete tasks, you mark them off by swiping your finger across them. The app keeps track of streaks, which encourages the user to maintain their desired habit.
The app is beautifully designed, and only $1.99. If it helps you change even one habit, it is priceless.