Another useful app might be a "Crimewatch" app. This would be an app that took a picture and emailed it to a central server in as few button pushed as possible.
The idea being that anybody in the city would take a photo whenever they saw something suspicious. The picture would be uploaded to a police server along with the time and GPS data. Then if a crime were committed, the police could check the server for photos near the crime scene at the time it was committed.