You can do whatever you like with works in the the public domain because they are no longer covered by copyright! The most common reason for works being in the public domain is because they are old enough that their copyright protections ended. Other reasons include:
- works that cannot be covered by copyright like: titles, names, weights and measures, ideas and facts, government works, processes and systems
- Works that were given over to the public domain by their creators.
To find works that are in the public domain, trying searching for the item you are looking for plus the phrase "public domain" with your preferred search engine. Other rich sources include the Library of Congress, the Smithsonian, the New York Times image archive, Project Gutenburg (for books) and even NDSU's own archives!
One way in which people are increasingly giving over part or all of the copyright protections covering their work and so turning it over to the public domain is Creative Commons. It was designed to let people be more creative, innovative and sharing of their work. Look for sites like Flikr's Creative Commons to find resources you can legally use. Searching online for the item you are looking for along with the phrase "creative commons" can help you to find these materials.
Copyright, fair use and public domain information courtesy of the Electronic Frontier Foundation's online curriculum, http://www.teachingcopyright.org/