The non-profit group Creative Commons is offering a Public Domain Mark (see above) for use on documents and files to declare them copyright-free.
Creative Common’s description: “Using the Public Domain Mark, you can mark a work that is free of known copyright restrictions and clearly convey that status. When applied properly, the PDM allows the work to be easily discovered, and provides valuable information about the work.”
Creative Commons is a nonprofit corporation “dedicated to making it easier for people to share and build upon the work of others, consistent with the rules of copyright.”
Tom Kelchner