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Public domain equivalent license are licenses that grant public-domain-like rights or/and act as waivers. They are used to make copyrighted works unrestricted usable by anyone; for instance to create public-domain software, which has all four freedoms but is not hampered by the complexities of attribution or license compatibility. The extremely permissive terms of public domain equivalent licenses meet the definition of copyfree licenses.
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Description
While real public domain makes licenses (for instance software licenses) unnecessary, as no owner/author is required to grant permission, there are licenses that grant public-domain-like rights. Reason for the existence of PD licenses (instead of waivers like anti-copyright notices) is that not all global jurisdictions allow to waive ("giving up rights") all rights away, while granting rights ("licensing rights") is almost everywhere possible. While there is no universally agreed-upon license, several licenses aim to release source code (or other works) into the public domain.
Licenses
In 2000 the WTFPL was released as public-domain-like license/waiver/anti-copyright notice especially for software. 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 the Creative Commons CC0 as a recommended license to dedicate content and software to the public domain. In June 2016 an analysis of the Fedora Project's software packages revealed the CC0 as the seventeenth most popular license.
The Unlicense, published around 2010, has a focus on an anti-copyright message. The Unlicense 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.