Works lacking Copyright right protection reside in the public domain. Works of this nature include United States Government publications, simple facts or other non-protectable works, or the Copyright has expired.
Cornell University has a great chart organizing and describing the categories of works in the Public Domain: http://copyright.cornell.edu/resources/publicdomain.cfm
Use the U.S. Copyright Office search tool to look up registered or expired works: http://www.copyright.gov/records/