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.
Yahoo's advanced search page allows you to limits you results to .gov materials:
Google's advanced search and advanced video search 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
(2) American Memory - items related to US history online.
(3) Motion Picture, Broadcasting, and Recorded Sound
(4) Moving Image Research Center
For more information about finding and using public domain materials, a highly recommended source is The Public Domain by Stephen Fishman. The Meriam Library has the 5th edition (2010). Fishman has also constructed a website of public domain related resources (last updated January 2008).
Meriam Library | CSU, Chico