The New York Times reported today on a powerful crime-fighting tool being used by the New York Police Department: total information awareness of where automobiles go on city streets, by their license plate numbers.

Thanks to a network of 108 fixed cameras and 130 cameras mounted on squad cars, the NYPD is able to keep a centralized record of the movements of individual cars throughout the city. This system was meant to protect New York from the horrors of terrorism, originally:
Though the imaging technology was conceived primarily as a counterterrorism tool, the cameras’ presence — all those sets of watchful eyes that never seem to blink — has aided in all sorts of traditional criminal investigations
Terrorism doesn't happen all that often, right? So it would be a waste to have this surveillance equipment sitting around doing nothing with all the license plate numbers it reads. The Times cited multiple manhunts where the cameras helped police find people they might not have found, or might not have found quite so soon.

But manhunts don't happen all the time either. On inspection, the Times story only had three examples in the past two years. So...
The cameras have provided clues in homicide cases and other serious crimes. But they have been used in lesser offenses, too. With them, stolen cars have been identified, located and returned. The cameras have uncovered unregistered vehicles and those with stolen license plates.
Unregistered vehicles. There is a citywide automated automobile-tracking system in place to catch unregistered vehicles.
More here: Scocca : New York's Robot Police Cameras Leave No Hiding Place for Terrorists, Criminals, People With Faulty DMV Paperwork