With the way copyright has been mangled in the last 35 years, public domain may not be safe - I have seen several people commenting that there is no way, since the copyright changes in the late 1970s, to affirmatively put your work in the public domain. Because of that risk, the WTFPL - Do What The Fuck You Want To Public License, http://sam.zoy.org/wtfpl/ , or an equivalent, is safer for potential users. Note that on the page I linked, it is pointed out that some non-Anglosphere countries didn't have a public domain in the first place.