David Amsden in The New York Times:

On the morning of Sunday, March 29, Sidney Torres was sipping an espresso in the kitchen of his mansion on the edge of the French Quarter when a jarring notification lit up his iPad and two iPhones. Pimps fighting with drug dealers and johns. Man has gun. Hurry. The message came from a neighbor 10 blocks away, on St. Louis Street, and was sent through a venture Torres started four days earlier: a private police patrol that could be summoned via mobile app.