Twenty One Day Habit was a short weekend project. The idea of the app is to check off doing a 'good habit' each day for 21 days. At the end of the 21 days you should have it engrained into your daily routines.
I've actually had some success with it, some of my friends even have told me it has actually helped them in their lives! It currently gets the most traffic of any of sites.
Try it out. The Facebook integration, simplicity and SMS integration are the features I'm very proud of.