Creative Commons now has a Public Domain Mark designed for designating works for which there is no known copyright for anywhere in the world. In a press release announcing the creation of the Public Domain Mark, Creative Common said,
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.
Creative Commons already has a license for creators to waive all rights for works that are currently covered by copyright in some jurisdiction or another: the CC0 license.