Public Domain is not a license and isn't legally accepted in some countries. So if you want to give stuff for free, it's better to just license it under MIT or BSD.
Public Domain Software
The term “public domain” refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it. - Stanford University definition
 All copyright holders must agree. Dedicating to Public Domain requires waiving of copyright causing any license to become null and void. This action is not retroactive in the sense that copies of previous versions retain original licenses.
 This invariably revokes Public Domain status of that copy going forward, regardless of license chosen. Other copies of previous versions of the work remain in Public Domain. i.e Forks of commits before license changes.
In practice, Public Domain status can be completely revoked from a work provided no previous copies are held by third-party. This might involve, for example, deleting relevant commits from all public git repositories to prevent someone from forking the project from that commit.
 Patents apply to inventions, not creative works. It is possible to patent inventions before using them in them in Public Domain work.
 Unless proven that it was unrightfully dedicated to Public Domain.
 Even though you can freely omit authorship, you cannot wrongfully claim authorship of work dedicated to Public Domain.
 Local laws apply.
In general, open-source advocates do not sympathise with Public Domain arguing that it revokes all copyright rights in open-source software allowing copies and derivatives to be licensed under closed-source licenses, which is true. They prefer permissive (MIT) or copyleft (GNU GPL) licenses which offer copyright protection and disallow changing of license.
Profitable Public Domain software
- SQLite - A free, reliable, portable and most widely deployed database engine in the world. While the database engine is in Public Domain, its automated test suit which is paramount to ensure reliability and performance has 700 times as much code than the engine is closed source. This strongly discourages any competition from copying the engine code when faced with that daunting task of writing all tests from scratch.
Public Domain implies waiving all copyright and thus Public Domain is not a license.
By definition, a software license assert some form of copyright and therefore no license is Public Domain.
Popular "Public Domain like" licenses
The Unlicense - Is a template for disclaiming copyright monopoly interest in software you've written; in other words, it is a template for dedicating your software to the Public Domain. It is not a license, per se, but rather the most accepted method of expressing that a project was dedicated to the Public Domain.
WTFPL (Do What the Fuck You Want To Public License) - Informal license that implies total freedom of use. Also cannot be considered Public Domain compatible as it includes copyright ownership.