How about this for an idea? Not a killer app, but a life saving one (pun intended) :P Should be super easy to build as well and has the potential to become famous, if presented well on the UI.
Every time someone smokes a cigarette, their life span is cut short by x minutes (I don't know the exact figure). So, your application can be used as a means to quit smoking. Each time someone smokes, they'll hit the "I smoked" button on the app and the app will tell them the number of minutes they just lost in their lives. Or even go a step further and show the cumulative number of days, someone lost in their life.
Mario Giambanco
Director of User Experience Development