For the past few years the Oakland police department has been using license plate reader (LPR) technology to record the license plates of cars. In response to a public records requests the city released the entire data set to Ars Technica and later posted 13 of the original 20 files to their open data portal.
The released LPR data may be searched and mapped at this site, oaklandlpr.com. The site uses Flask and is backed by a MySQL database containing 2,738,523 LPR records. A sample search for a license plate belonging to an Oakland Police patrol car turns up 240 records.
UPDATE: I had to retire this website so the links no longer work. I used it as an opportunity to experiment with AWS, and once the trial period ended I couldn’t justify the expense of hosting it.