Public domain works are not protected by copyright either because their copyright term has expired, or they fall within a category of works that are not subject to copyright law. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission.
An important caveat regarding public domain material is that collections, new editions, and derivative works of public domain material may all be protected by copyright. With collections, an author could collect public domain works in a book or display them on a website, and the collection as a whole could be protected by copyright, even though individual works within it are not.
There are four common ways that works arrive in the public domain:
• the copyright has expired.
In general, the copyright term for a work created in the United States after 1977 (that is not a work made for hire) is the life of the author plus 70 years.
All works published in the United States before 1926 are in the public domain.
Works published between 1926 and 1978 may be protected by copyright if they were published with notice and the copyright was renewed. There are many other scenarios and sets of circumstances that affect the copyright term of these earlier works. Help with these complicated rules, and other scenarios can be found in this chart and in the U.S. Copyright Office's circular on the Duration of Copyright (PDF). You may also use the Copyright Genie or U.S. Copyright Office Records to help determine if a work is in the public domain.
• the owner was required to renew the copyright and failed to do so.
At certain points in history copyright holders were required to renew their copyright if they wished to keep their works from entering the public domain. Renewal is no longer required to retain copyright for the full term allowed for works created after 1978.
• the owner deliberately places it in the public domain.
To place an item in the public domain intentionally, a creator of a work would need to state that intent explicitly. If no such statement has been made, assume the work is protected by copyright. (Compare alternative licenses like Creative Commons.)
• copyright law does not protect this type of work.
Copyright will not protect the titles of a book or movie, nor will it protect short phrases such as "Make my day" (though trademark protection may apply). Copyright protection also doesn't cover facts, ideas or theories, although it may protect the expression of those ideas. Any work created by a U.S. government employee or officer is in the public domain, provided that the work is created in that person's official capacity. This applies only to U.S. government works, not the works of other national or state governments.
Works that are in the public domain can be found almost anywhere so this is far from an exhautive list. The following is merely a selection of sites that have large collections of content in the public domain.