I like the idea of citizens being part on scientific researches. You feel good knowing that you are helping your city with an easy action, maybe using your phone as a sensor or just observing.

The project Smart Citizen is a good example of air quality sensing, but it can gather any kind of information, like "Onde Fui Roubado" which people can share places they were stolen and it's possible to build the map of dangerous areas of the city (therefore areas police authorities should enforce its actions).