The Prelinger Archive
There is a little of everything here and most of the films are from the 1920s through the 1960s. If you need atmosphere shots from this period of time this is the place to look because it is easy to find shots of people, streets, cars and trucks, trains and planes, representative of these years. There are lots of industrials and business films produced by the Jam Handy organization, educational films by Coronet, government films, newsreels, commercials and even a section of home movies.
The library is very well indexed and most of the films are available in a variety of download formats.
Every video producer should spend an afternoon browsing this library and making notes of potential footage that would be useful on upcoming projects and that would serve your client base.