Links to Copyright-free and Public Domain Video

the Prelinger Archives : all sorts of stuff, thousands of classic TV ads, government propaganda films, cartoons and what have you.
"You are warmly encouraged to download, use and reproduce these films in whole or in part, in any medium or market throughout the world."
Try Self Preservation In An Atomic Attack or In My Merry Oldsmobile 1932
http://www.archive.org/details/prelinger

Open-video.org: includes such classics as A is for Atom (1953) and Brink of Disaster (Part II) (1972) - How 1960s activism 'threatens' American moral, religious and ethical principles.
http://www.open-video.org/

Public Domain Comedy Video
http://www.pdcomedy.com/

WGBH Sandbox : organized loosely by subject and type. Some cool stuff here, all available for download to cut up, remix, whatever
http://lab.wgbh.org/sandbox/