What is Public Domain
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.
Where to find Public Domain Content
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: http://search.yahoo.com/search/options?fr=fp-top&p=
Google's advanced search http://www.google.com/advanced_search can be limited by "usage rights." 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.
Project Gutenberg offers over 42,000 free ebooks: choose among free epub books, free kindle books, download them or read them online.
Free Software Foundation (FSF)
Public Domain Tools (from Creative Commons)
Creative Commons public domain tools enable authors and copyright owners who want to dedicate their works to the worldwide public domain to do so, and facilitate the labeling and discovery of works that are already free of known copyright restrictions.