Williamsburg: the Story of a Patriot (1957)

Williamsburg: the Story of a Patriot, an orientation film produced by Paramount Pictures and the Colonial Williamsburg Foundation in 1957, has the distinction of being the longest-running motion picture in history, having been shown continually in the Colonial Williamsburg Visitor Center for over five decades. On September 20, 2002, it was seen by the 30 millionth viewer.

More at Wikipedia.