Unfortunately, it seems like there aren't any OSI approved licenses for software authors that are prepackaged and ready for use with public domain software. The OSI actually doesn't have such a license because it believes it can't correctly define what Public Domain means to the author within the license itself. I disagree with that assessment.
Having researched numerous licenses over the years, I'm very comfortable with various licenses. The MIT license is, in my non-legal opinion (because I'm not a lawyer), the most liberal open source license that's as close the public domain without it actually being public domain. It basically says, "Hey, you can do whatever you want with this software just don't sue me if it causes harm. However, please note that I own the code and you have to include this license and my claim to copyright somewhere in your software." That last little bit is a sticking point if you don't WANT to claim copyright because you want it to be in the public domain.
Toward this end, here is my best attempt to satisfy the concerns of OSI regarding public domain with a modified MIT license:
Modified MIT License for Public Domain software Public Domain or legal equivalent Original authorship by [authors] (the "Authors") in [year] Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
The first line of the actual agreement ("Public Domain or legal equivalent") is intended to be treated as a title for the agreement, having legal impact wherever it applies (e.g. United States Copyright Law clearly defines what Public Domain means). If a locale has no such definition, the first line will most likely be overlooked and the second line ("Original authorship by [authors] (the "authors") in [year]") says who is the actual owner of the copyright (i.e. a legal fallback mechanism). "Original authorship" is a clever avoidance of the legally defined word "copyright" in many locales.
The first paragraph therefore will only technically apply to those locales where copyright law has no official definition of Public Domain. However, some or all of the terms may apply regardless. In essence, it clarifies the intentions of the author in regards to their hold on copyright. Should copyright still apply, attribution is included with the license to indicate who the claimants actually are. Obviously someone with intention to place their software into the Public Domain has no intention to ever claim ownership of their copyright in the first place, which effectively neuters any legal effect that the first paragraph might have. The definition of Public Domain is therefore clearly defined by the first paragraph. Whenever and wherever there may be doubt, define what you mean.
The second paragraph is the "covering your legal rear" paragraph. Of the two paragraphs, this is the most important one because it protects the authors from many types of lawsuits. However, wherever copyright law may still apply (and even those locales where it doesn't), the combination with the first paragraph adds extra protection by clearly specifying that the author allows the software to be used for any purpose, thus removing all legal liability (wherever legally allowed). I removed the words that referenced "copyright holders" since, by definition of public domain, there are no copyright holders. However, the use of '(the "Authors")' in the legal fallback mechanism used earlier means that the word 'AUTHORS' is tied to those who hold the copyright (if any). Even if copyright doesn't apply, the word 'AUTHORS' in that specific location protects the authors from legal liability for the software. This really wraps up the entire package, puts a fancy bow on it, and tells lawyers to go away.
It is my opinion that this license represents the closest to public domain that we as software developers can get legally worldwide while staying really far away from liability lawsuits.
Permission is hereby granted, free of charge, to any person obtaining a copy of this license (the "License", to deal in the License without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the License, and to permit persons to whom the License is furnished to do so.
THE LICENSE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE LICENSE OR THE USE OR OTHER DEALINGS IN THE LICENSE.
Hey, gotta cover my legal rear when writing licenses too. Interestingly, the license itself is a kind of circular reference.