|FSF approved||Yes |
|OSI approved||Yes |
|GPL compatible||Yes |
|Linking from code with a different licence||Yes|
The Unlicense is a public domain equivalent license for software which provides a public domain waiver with a fall-back public-domain-like license, similar to the CC Zero for cultural works.  It includes language used in earlier software projects and has a focus on an anti-copyright message.  
The text of the Unlicense is as follows: 
This is free and unencumbered software released into the public domain.
Anyone is free to copy, modify, publish, use, compile, sell, or distribute this software, either in source code form or as a compiled binary, for any purpose, commercial or non-commercial, and by any means.
In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain. We make this dedication for the benefit of the public at large and to the detriment of our heirs and successors. We intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.
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.
For more information, please refer to <http://unlicense.org/>
The Free Software Foundation states that "Both public domain works and the lax license provided by the Unlicense are compatible with the GNU GPL." However, for dedicating software to the public domain it recommends CC0 over the Unlicense, stating that CC0 "is more thorough and mature than the Unlicense". 
Google does not allow contributions to projects under public domain equivalent licenses like the Unlicense (and CC0), while allowing contributions to 0BSD licensed and US government PD projects. 
Notable projects that use the Unlicense include youtube-dl,  Second Reality,  and the source code of the 1995 video game Gloom . 
In a post published on January 1 (Public Domain Day), 2010, Arto Bendiken, the author of the Unlicense, outlined his reasons for preferring public domain software, namely: the nuisance of dealing with licensing terms (for instance license incompatibility), the threat inherent in copyright law, and the impracticability of copyright law. 
On January 23, 2010, Bendiken followed-up on his initial post. In this post, he explained that the Unlicense is based on the copyright waiver of SQLite with the no-warranty statement from the MIT License. He then walked through the license, commenting on each part. 
In a post published in December 2010, Bendiken further clarified what it means to "license" and "unlicense" software. 
In December 2010, Mike Linksvayer, the vice president of Creative Commons at the time, wrote in an identi.ca conversation "I like the movement" in speaking of the Unlicense effort, considering it compatible with the goals of the CC Zero (CC0) license, released in 2009.   On January 1, 2011, Bendiken reviewed the progress and adoption of the Unlicense, saying it was "difficult to give estimates of current Unlicense adoption" but there were "many hundreds of projects using the Unlicense". 
In January 2012, when discussed on OSI's license-review mailing list, the Unlicense was brushed off as a crayon license. In particular, it was criticized for being possibly inconsistent and non-standard, and for making it difficult for some projects to accept Unlicensed code as third-party contributions; leaving too much room for interpretation; and possibly being incoherent in some legal systems.    A request for legacy approval was filed in March 2020,  which led to a formal approval in June 2020, with an acknowledgement of a "general agreement that the document is poorly drafted". 
In 2015, GitHub reported that approximately 102,000 of their 5.1 million licensed projects (2% of licensed projects on GitHub.com) used the Unlicense. 
Until 2022, the Fedora Project recommended CC0 over the Unlicense because the former is "a more comprehensive legal text".  However, in July 2022, the CC0 license became unsupported and software to be released in the Fedora distribution must not be under CC0, due to CC0 not waiving patent rights. 
The MIT License is a permissive free software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts only very limited restriction on reuse and has, therefore, high license compatibility.
The Apache License is a permissive free software license written by the Apache Software Foundation (ASF). It allows users to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software under the terms of the license, without concern for royalties. The ASF and its projects release their software products under the Apache License. The license is also used by many non-ASF projects.
A Creative Commons (CC) license is one of several public copyright licenses that enable the free distribution of an otherwise copyrighted "work". A CC license is used when an author wants to give other people the right to share, use, and build upon a work that the author has created. CC provides an author flexibility and protects the people who use or redistribute an author's work from concerns of copyright infringement as long as they abide by the conditions that are specified in the license by which the author distributes the work.
A software license is a legal instrument governing the use or redistribution of software. Under United States copyright law, all software is copyright protected, in both source code and object code forms, unless that software was developed by the United States Government, in which case it cannot be copyrighted. Authors of copyrighted software can donate their software to the public domain, in which case it is also not covered by copyright and, as a result, cannot be licensed.
License-free software is computer software that is not explicitly in the public domain, but the authors appear to intend free use, modification, distribution and distribution of the modified software, similar to the freedoms defined for free software. Since the author of the software has not made the terms of the license explicit, the software is technically copyrighted according to the Berne convention and as such is proprietary.
A permissive software license, sometimes also called BSD-like or BSD-style license, is a free-software license which instead of copyleft protections, carries only minimal restrictions on how the software can be used, modified, and redistributed, usually including a warranty disclaimer. Examples include the GNU All-permissive License, MIT License, BSD licenses, Apple Public Source License and Apache license. As of 2016, the most popular free-software license is the permissive MIT license.
The Eclipse Public License (EPL) is a free and open source software license most notably used for the Eclipse IDE and other projects by the Eclipse Foundation. It replaces the Common Public License (CPL) and removes certain terms relating to litigations related to patents.
This comparison only covers software licenses which have a linked Wikipedia article for details and which are approved by at least one of the following expert groups: the Free Software Foundation, the Open Source Initiative, the Debian Project and the Fedora Project. For a list of licenses not specifically intended for software, see List of free-content licences.
The ISC license is a permissive free software license published by the Internet Software Consortium, now called Internet Systems Consortium (ISC). It is functionally equivalent to the simplified BSD and MIT licenses, but without language deemed unnecessary following the Berne Convention.
Public-domain software is software that has been placed in the public domain, in other words, software for which there is absolutely no ownership such as copyright, trademark, or patent. Software in the public domain can be modified, distributed, or sold even without any attribution by anyone; this is unlike the common case of software under exclusive copyright, where licenses grant limited usage rights.
The WTFPL is a permissive free software license. As a public domain like license, the WTFPL is essentially the same as dedication to the public domain. It allows redistribution and modification of the work under any terms. The title is an abbreviation of "Do What The Fuck You Want To Public License".
Public-domain-equivalent license are licenses that grant public-domain-like rights and/or act as waivers. They are used to make copyrighted works usable by anyone without conditions, while avoiding the complexities of attribution or license compatibility that occur with other licenses.
License proliferation is the phenomenon of an abundance of already existing and the continued creation of new software licenses for software and software packages in the FOSS ecosystem. License proliferation affects the whole FOSS ecosystem negatively by the burden of increasingly complex license selection, license interaction, and license compatibility considerations.
Mike Linksvayer is an intellectual freedom and commons proponent, known as a technology entrepreneur, developer and activist from co-founding Bitzi and leadership of Creative Commons. He is GitHub's Policy Director.
A free-software license is a notice that grants the recipient of a piece of software extensive rights to modify and redistribute that software. These actions are usually prohibited by copyright law, but the rights-holder of a piece of software can remove these restrictions by accompanying the software with a software license which grants the recipient these rights. Software using such a license is free software as conferred by the copyright holder. Free-software licenses are applied to software in source code and also binary object-code form, as the copyright law recognizes both forms.
BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft licenses, which have share-alike requirements. The original BSD license was used for its namesake, the Berkeley Software Distribution (BSD), a Unix-like operating system. The original version has since been revised, and its descendants are referred to as modified BSD licenses.
A free license or open license is a license which allows others to reuse another creator’s work as they wish. Without a special license, these uses are normally prohibited by copyright, patent or commercial license. Most free licenses are worldwide, royalty-free, non-exclusive, and perpetual. Free licenses are often the basis of crowdsourcing and crowdfunding projects.
A public license or public copyright licenses is a license by which a copyright holder as licensor can grant additional copyright permissions to any and all persons in the general public as licensees. By applying a public license to a work, provided that the licensees obey the terms and conditions of the license, copyright holders give permission for others to copy or change their work in ways that would otherwise infringe copyright law.
Software relicensing is applied in open-source software development when software licenses of software modules are incompatible and are required to be compatible for a greater combined work. Licenses applied to software as copyrightable works, in source code as binary form, can contain contradictory clauses. These requirements can make it impossible to combine source code or content of several software works to create a new combined one.
There is general agreement that the document is poorly drafted. It is an attempt to dedicate a work to the public domain (which, taken alone, would not be approved as an open source license) but it also has wording commonly used for license grants. There was some discussion about the legal effectiveness of the document, in particular how it would operate in a jurisdiction where one cannot dedicate a work to the public domain. The lawyers who opined on the issue, both US and non-US, agreed that the document would most likely be interpreted as a license and that the license met the OSD. It is therefore recommended for approval.
Fedora recommends use of CC-0 over this license, because it is a more comprehensive legal text around this tricky issue. It is also noteworthy that some MIT variant licenses which contain the right to "sublicense" are closer to a true Public Domain declaration than the one in the "Unlicense" text.
anybody affixing a licensing statement to open-source software is guilty of either magical thinking or of having an intention to follow up on the implied threat
@bendiken surely there's a better name than copyfree, but I like the movement and look fwd to your roundup.
In case it's of interest, I'm engaged in an ongoing Identi.ca conversation with Mike Linksvayer, the vice president of Creative Commons […] In short, the folks at Creative Commons are aware of the Unlicense initiative, and apparently supportive of it.
1 MIT 44.69%, 2 Other 15.68%, 3 GPLv2 12.96%, 4 Apache 11.19%, 5 GPLv3 8.88%, 6 BSD 3-clause 4.53%, 7 Unlicense 1.87%, 8 BSD 2-clause 1.70%, 9 LGPLv3 1.30%, 10 AGPLv3 1.05% (30 million × 2% × 17% = 102k)