Public-domain-equivalent license

No permission or license is required for a work truly in the public domain, such as one with an expired copyright; such a work may be copied at will. Public domain equivalent licenses exist because some legal jurisdictions do not provide for authors to voluntarily place their work in the public domain, but do allow them to grant arbitrarily broad rights in the work to the public.

The licensing process also allows authors, particularly software authors, the opportunity to explicitly deny any implied warranty that might give someone a basis for legal action against them. While there is no universally agreed-upon license, several licenses aim to grant the same rights that would apply to a work in the public domain.

Licenses

In 2000, the "Do What the Fuck You Want To Public License" (WTFPL) was released as a public-domain-equivalent license for software. It is distinguished among software licenses by its informal style and lack of a warranty disclaimer. In 2016, according to Black Duck Software, the WTFPL was used by less than 1% of FOSS projects.

In 2009, Creative Commons released CC0, which was created for compatibility with jurisdictions where dedicating to public domain is problematic, such as continental Europe. This is achieved by a public-domain waiver statement and a fall-back all-permissive license, for cases where the waiver is not valid. The Free Software Foundation and the Open Knowledge Foundation approved CC0 as a recommended license to dedicate content to the public domain. The FSF and the Open Source Initiative, however, do not recommend the usage of this license for software due to inclusion of a clause expressly stating it does not grant patent licenses. In June 2016 an analysis of the Fedora Project's software packages placed CC0 as the 17th most popular license.

The Unlicense software license, published around 2010, offers a public-domain waiver text with a fall-back public-domain-like license, inspired by permissive licenses but without an attribution clause. In 2015 GitHub reported that approximately 102,000 of their 5.1 million licensed projects, or 2%, use the Unlicense.

The BSD Zero Clause License removes half a sentence from the ISC license, leaving only an unconditional grant of rights and a warranty disclaimer. It is listed by the Software Package Data Exchange as the Zero Clause BSD license, with the SPDX identifier 0BSD. It was first used by Rob Landley in Toybox and is OSI-approved.

The MIT No Attribution License, a variation of the MIT License, was published in 2018 and has the identifier MIT-0 in the SPDX License List.