Here are a few ways to find works in the public domain:
Use the Search Box on the Creative Commons web site.
Use Google's Advanced Search (select Settings on the bottom right side of the screen).
Select your choice of Usage Rights filter on the Advanced Search screen.
Try the Directory of Open Access Journals (DOAJ).
Look for institutional repositories on their web sites, Google search for them. Digital Commons has a listing as well:
Works that may be freely used, without copyright restrictions, are in the public domain.
As stated by the US Copyright office, here are the four main instances:
Generic information such as facts, numbers and ideas.
Works whose copyrights have lapsed over time or whose copyright holders have failed to renew a registration (a requirement that applies to works created before 1978).
Works published before March 1989 that failed to include a proper notice of copyright.
Works created by the U.S. federal government.
Stanford University's Guide states:
the copyright has expired
the copyright owner failed to follow copyright renewal rules
the copyright owner deliberately places it in the public domain, known as “dedication,” or
copyright law does not protect this type of work.
**Copyright has expired for all works published before 1923 and is now in the public domain.
In addition, some authors allow others to freely use their works.
This would be through Creative Commons and Open Access.
This allows a work creator to license their work under a Creative Commons license. Creators do this to encourage the use of their work by others, without copyright restrictions.
This then allows the work to be freely shared with others, without copyright restrictions or fees, so long as the people who use the work give credit to the author/creator and follow the license agreement selected by the creator.
For more information, please see the Creative Commons web site.
The publication of works in open access journals and in institutional repositories that allows the use of the works without copyright restrictions.
Resources, books, and tools for education, that are available free of copyright restriction, through the internet.
Data that is made available free of copyright restrictions through the internet.
More information on open access, open education and open data, see SPARC.