Items in the Public Domain
Intellectual Property Information Center at Kansas State University Online Tutorial on Copyright Basics
Works which are not protected by copyright are referred to as being in the "public domain." Public domain items include most documents written by the federal government -- which are not protected by copyright -- and those works which have had their copyright term expire (once copyright expires it cannot be retrieved).
There is no central listing of works in the public domain. The Copyright Office can do a search on a work's title, author's name or the claimant's name to see if an item has entered the public domain, however, the current charge for this starts at $65 an hour.