LOTS AND LOTS of public domain movies
http://www.archive.org/movies/search...&searchAll=yes

If you have A LOT of hard drive space, you can try using hhtrack and set robots option off and visit
ftp://ftp.archive.org/pub/movies/ to download a lot of the mpg files.

As far as I know, the Prelinger archive is all stuff that's old enough to now be public domain.