The Cleveland Air Route Traffic Control Center (ZOB) is the busiest of the 22 Air Route Traffic Control Centers in the United States and the busiest such facility in the world.
The Cleveland ARTCC is one of the most transitioned air traffic control centers in the country, and oversees the airspace over portions of Maryland, Michigan, New York, Ohio, Pennsylvania, West Virginia, as well as the southernmost portion of Ontario, Canada.
Also:
http://en.wikipedia.org/wiki/ARTCC