The term Public Domain refers to materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it." Stim (2000). Most works of the Federal government are automatically in the Public Domain.
Stim, R. (2000) Getting Permission: How to License and Clear Copyrighted Materials Online & Off.
For Federal Government public domain materials you can search in the departments and agencies websites.
The USA.gov website has an image/photo search tool that searches many agencies at once.
Google's advanced search http://www.google.com/advanced_search and advanced video search http://video.google.com/videoadvancedsearch results can be limited to government materials by entering .gov in the domain box.
The National Science Digital Library is a rich site for math, science, and engineering.
Library of Congress is rich in materials for those in the humanities.
(1) Digital Collections and Services: http://www.loc.gov/library/libarch-digital.html
(2) American Memory has more than 9 million items related to US history online, according to Stephen Fishman (2012):
(3) Motion Pictures in American Memory http://memory.loc.gov/ammem/browse/ListSome.php?format=Motion+PictureMotion
(4) Picture, Television Reading Room http://www.loc.gov/rr/mopic/
For more information about Public Domain materials, a highty recommended source is Stephen Fishman's book, "The Public Domain," published by Nolo Press. The Meriam Library has the 5th edition (2010). http://opac.csuchico.edu/record=b1926657~S5 Fishman has constructed a webpage with links at: http://copyrightfree.blogspot.com/