Form their website:
Today, Creative Commons announces the release of its Public Domain Mark, a tool that enables works free of known copyright restrictions to be labeled in a way that allows them to be easily discovered over the Internet. The Public Domain Mark, to be used for marking works already free of copyright, complements Creative Commons’ CC0 public domain dedication, which enables authors to relinquish their rights prior to the expiration of copyright.
“The Public Domain Mark is a further step on the path towards making the promise of a digital public domain a reality,” said Michael Carroll, a founding board member of Creative Commons and a law professor at American University.
Europeana—Europe’s digital library, museum and archive—is the first major adopter of the Public Domain Mark. Europeana estimates that by mid-2011, the Public Domain Mark will be used in connection with millions of out-of-copyright works made available through its portal. … The Public Domain Mark in its current form is intended for use with works that are free of known copyright around the world, primarily old works that are beyond the reach of copyright in all jurisdictions. We have already started mapping the next phases of our public domain work, which will look at ways to identify and mark works that are in the public domain in a limited number of countries.