This is as exciting as when I figured out how to download free loops for Garage Band.
Check out the Moving Image Archive, which is like heaven for film geeks.
It's a library of free movies, films, and videos, ranging from full-length classic films to archival footage of street scenes in American cities through the centuries, public service announcements to art and music, cultural and academic films - and almost all of it is free to download, so you can use these moving images to make your own films!
I have already downloaded little weird movie commercials for pizza and candy and spliced them into a disjointed music video. This stuff is so fun.