Around here, its almost exclusively Remington 870 Police Magnums in both 18.5 and 20 inch barrel lengths with Speed Feed III stocks and bead sights. However, Mossberg 590A1's are used by several of the smaller agencies. One of the small town/suburban agencies is extremely well funded and uses Benelli M4's in 18.5 and 14 inch barrel lengths. They have a serious love affair with the "latest and greatest" which explains the HK G36's/416's/UMP's in the hands of a PD that is responsible for policing a small town of 18,000 persons....
By far, the 870 and Mossberg 500 series are the most popular shotguns in American LE. Occasionally, smaller agencies might have "left over" Winchester 1200/1300 Defenders, Ithaca M37's or even Histandard Flite Kings. The latter two are still seen in service with the LAPD.