I know CC0 and the WTFPL for example exist to grant all rights associated with the public domain, without placing it in the public domain, because this is difficult, or in some cases impossible. I know it is impossible in Norway and Germany.
Why is this difficult, and how can I release some project effectively into the public domain, for jurisdictions that have such a concept?
If it varies greatly per jurisdiction, how can I do it for the US?
TL;DR
The safest way to signal your intent to give up all rights on your work currently is CC0 in most jurisdictions. The safest way to use works in the public domain is to still do it like moral rights apply, even if the copyright in your country doesn't know the concept.
First of all, most jurisdictions have some concept that is more or less similar to public domain (in germany for instance 'Gemeinfreiheit'). But only some countries allow to explicitly assign a work to public domain. So in many jurisdictions a work only becomes public domain, if the time for exclusive rights expire (70 years after authors death in many countries).
To go more into detail here, the Wikipedia-article covers 4 ways a work can become public domain.
Works not covered by copyright law
This is for works, that for some reasons are not covered by copyright laws. This is working in most jurisdictions, although there might be different rules what is covered and what not.
Expiration of copyright
That's what I wrote above, pretty much all jurisdiction have a limit on how long a copyright can be claimed. After that the work can be counted as public domain.
Government works
This is varying very much between jurisdictions. Which of governmental works (or works of other public bodies) is counted as public domain is very dependent on local laws.
Dedicating works to the public domain
This is the case the question implicitly asks. The author explicitly applies his works to public domain. Wikipedia writes here:
Few if any legal systems have a process for reliably donating works to the public domain. They may even prohibit any attempt by copyright owners to surrender rights automatically conferred by law, particularly moral rights.
As can be seen, this is the difficult part. Countries like Germany know the concept of moral rights, that you cannot give up. These cover the rights of the author for attribution or that a work isn't used to denigrate the author.
Moral rights
It has to be noted, that many countries maintain the moral rights even after the commercial rights are expired. So in these countries you still may not use a work of a long dead author for example without proper attribution. In these cases you can say that it is impossible for a work to really properly become part of the public domain. The Berne Convention includes moral rights, although the final ratification still differs strongly even between signatories.
Even for the US, that hasn't the concept of moral rights, it still might be against other laws to violate the moral rights, for instance laws against defamation.
How can I dedicate a work to public domain
You can simply declare, that you put your work under public domain. This may even work in countries that haven't the concept: I remember a law-suit, there the court accepted the public domain dedication as the authors intent to give up his rights.
Also there are some licenses (you name CC0 and WTFPL) that work in that way and give rights similar to what Public Domain would mean. CC0 seems a pretty safe way to make this work in most jurisdictions. Even if your country allows to dedicate a work to public domain, it may be safer to use CC0, as a user in another country might still not be allowed to use your work as PD.
Still, be it a declaration of public domain or CC0, you will keep your moral rights in countries where it applies. So it may be safe to use such works still with proper attribution and not use it in an abusive way.
TL;DR Things like CC0, Unlicense and WTFL create unexpeced legal hazards in some jurisdictions. The MIT License does not really put anything in the public domain, but it the safest license to use if you want to offer your stuff with as few restrictions as possible in a world-wide setting. In the USA, CC0 can be used, but I do not recommend it for software.
Why is this [to place software in the public domain] difficult?
The difficulty of placing software in the public domain in the public domain is closely related to something called "moral rights" that originated as civil law in the Napoleonic Code (1804, France) under the name of droit d’auteur (literally: author's rights).
Jurisdictions with strong moral rights have a tradition of making it near impossible to dedicate anything to the public domain.
Moral rights include the right of attribution, and the right of integrity (but the exact list of exactly what is protected by moral rights varies between jurisdictions).
In the Norwegian copyright act, moral rights are described in the first parts of § 3, while the third part says that these rights cannot be waived. Here is a direct quote (translated by me):
The author may not waive his rights under the first and second paragraphs, unless the use of the work in question is limited in nature and extent.
The "limited in nature and extent" addendum allows ghostwriting.
However part four states than under no circumstances can the right of integrity be waived. Quote (translated by me):
This right may not be waived by the author.
To compound the problem, moral rights are forever in Norway. In Germany, an author can commit suicide and then wait for 70 years to dedicate his works to the public domain. In Norway (and a few other countries in northern Europe), this option is not available.
how can I release some project effectively into the public domain, for jurisdictions that have such a concept?
I do not think that this is possible for all jurisdictions.
The most professional (as in: written by lawyers, not kids) attempt to create a legal tool to enable authors to dedicate their work to the public domain is CC0.
IMNSHO, while well intended, it fails, and its use creates a legal hazard in several jurisdictions (including Norway, where I live).
OSI seems to think so as well. In 2012, CC0 was submitted to OSI for review. After facing harsh criticism, the application was withdrawn.
(One of better known legal hazards of CC0 is its infamous patent clause, which I discuss in this answer.)
Another license that is supposed to do this is the Unlicense. AFAIK, it has never been submitted to OSI for review, but if it had, I am pretty sure it would have been summarily rejected. OSI's co-founder Bruce Perens call the the Unlicense a "crayon license", adding:
The point is that such things have done serious harm to developers when they've not behaved as expected in court. Thus, naïvely propagating one could be considered to be in the category of irresponsible acts that can damage others.
Personally, I believe that it is unrealistic to aim for the "public domain" if your audience is world-wide. There are too many local quirks in copyright and a one-size-fits-all legal tool to put something in the "public domain" may have some totally unexpected (and harmful) side effects.
For instance, in Norway, publicly performing recorded music distributed under CC0 requires you to pay a levy to GRAMO (a collecting society monopoly authorized under the Department of Culture), but doing the same with music distributed under CC-BY is free. Yes, this consequence of the use of this license is totally absurd, but some copyright laws (at least in Norway) are.
My recommendation is to stay clear of anything with pretense to put anything in the public domain, as well as "crayon" licenses such as "WTFL" and "Unlicense".
MIT
A license that will put as few restrictions as possible on the use of your project, while not creating legal hazards for yourself and your users, is the MIT (aka. Expat) license.
The MIT license does not abandon moral rights. Since they're not mentioned, the jurisdiction's default civil law applies.
The MIT license is also much more aligned with a "traditional" take on copyright. If you use the MIT license, authorship is not abandoned, but specific (normally reserved) rights are explicitly licensed to the licensee. This model fits much better into the legal framework of civil law jurisdictions than the much less conventional public domain dedications that is the gist of CC0, WTFPL and Unlicense.
If it varies greatly per jurisdiction, how can I do it for the US?
In a localized case, including the US, there is usually some recognized way to do this.
As for USA, you may consider using CC0 if you don't mind having your downstream recipients worry about submarine-patents. It was written by US lawyers, and it fits the legal situation in the USA to a tee - but it is really for artistic and literary works. IMNSHO, it should not be used for software.
Let me add that I think CC0 is pretty safe way of putting something in the public domain in common law jurisdictions, but much less safe to use in civil law jurisdictions.
Placing your work in the public domain is impossible in many countries and even illegal in some.
The Unlicense license will place your work in public domain for people who live in a country where it's possible, and license the work as closely as possible to public domain everyone else.
That's my preferred approach.
One of the most ubiquitous works of software, SQLite, is in the public domain with no license at all. That is highly unusual in the world of software.
Some wise people are behind SQLite, so I think you could do like they did and do the no licensing option to have your software live in the PD.
Although, the no-license approach probably isn't a good idea. I would follow the industry norm of getting a license as the other replies here discuss in great detail.
Also, wanted to post this in part to highlight that public domain per se, may be what you asked for, but not what you intended. It can be a loaded word.
