The Internet Archive is this neat site that archives web pages online. But I was poking around today and found they have a really neat online archive of old movies from the Prelinger Archives. Pretty neat. From the site:

This collection contains movies that the Prelinger Archives has digitized and donated to the Internet Archive. The films focus mainly on everyday life, culture, industry, and institutions in North America in the 20th century.

This is the first time that most of the films have been available to the public. They are "open source movies" -- available for viewing at no cost and with few restrictions. About 1254 films are now online.