US Federal government work (but not that of contractors, nor of state or local governments) is probably the largest source of public domain content by far. See US federal government websites and public domain and Wikipedia:Copyright status of work by the U.S. government.
You can search for Public Domain work using the box below. Results are mostly public domain (we're improving the search engine, but can never guarantee the results). Take your own precautions before using the material. See Appropedia's Public Domain Search for more information.