5. Pilgrim Psychiatric Center, Brentwood, New York
The facility steadily grew throughout its heyday, essentially becoming its own little town. The institution had its own police department, fire department, court, power plant, and its own Long Island Railroad station, among other things.
The Pilgrim State Psychiatric Center remains operational today, yet drastically smaller than it once was. Much of the land has been sold and converted for other uses, while some of the buildings have remained intact but abandoned.