I found another source of public domain video: Open-video.org. Whereas the Prelinger Archives is a collection of industrial films, this site is a general effort to archive video of all types.
If, for example, you’re curious about Thomas Edison’s short films from the early 20th century, you can find them there.
At the moment, the site is probably of more interest to historians, die-hard film fanatics, and aspiring moviemakers than to people seeking free movies to watch instead of heading to Blockbuster. It’ll be a while before it’s practical to download It’s a Wonderful Life in its entirety, and modern viewers are spoiled by recent video technology. I’m sure that Edison’s films had plenty of ooh-ahh factor in 1905, but by modern standards, the camera was shaky, the lighting inadequate, and defects in the film media itself jump out like the pops on an old vinyl record.
Most of the films are public domain, and it’s easy to find the usage terms on the site for each film available there. I’ve already got some ideas for things I can do with some of the footage from the site.