The Library of Congress has over 300 free movies, clips, promotional and documentary films from 1890 to 1999, through the National Screening Room collection.
The National Screening Room is a collection of films hosted by the Library of Congress. The collection lists films about America's past that are considered "culturally, historically or aesthetically significant" and have been archived by the Library of Congress. Most of the movies are public, and available for streaming or download.
The available collection can be sorted by title, date, original format and electronic format, geographic location and theme. There are currently 336 tracks in the collection, and new ones are added each month.
In addition to this collection, the US Congress library also has other collections containing songs and videos. A list of these you can find here.