If an "intellectual work" (a song, a book, a computer program, ...) is in the public domain (PD), it has no "owner", meaning no one has any exclusive rights (such as copyright or patent) over the work, no one can dictate how and by whom such work can be used and so anyone can basically do anything with such work (anything that's not otherwise illegal of course).
LRS highly supports public domain and recommends programmers and artists put their works in the public domain using waivers such as CC0.
Public domain is the ultimate form of freedom in the creative world. In public domain the creativity of people is not restricted. Anyone can study, remix, share and improve public domain works in any way, without a fear of being legally bullied by someone else.
Public domain is NOT the same thing as free (as in freedom) software, free culture or freeware (gratis, free as in beer) software. The differences are these:
This is not a trivial question, firstly because the term public domain is not clearly defined: the definition varies by each country's laws, and secondly because it is non-trivial and sometimes very difficult to assess the legal status of a work.
Corporations and capitalism are highly hostile towards public domain and try to destroy it, make it effectively non-existing, as to eliminate "free" works competing with the consumerist creations of the industry. Over many years they have pushes towards creating laws that make it extremely difficult and rare for works to fall into public domain.
Sadly due to these shitty laws most works created in latest decades are NOT in the public domain because of the copyright cancer: copyright is granted automatically, without any registration or fee, to the author of any shitty artistic creation, and its term lasts mostly for the whole life of the author plus 70 years! In some countries this is life + 100 years. In the US, copyright lasts 96 years from the publication of the work (every January 1st there is so called public domain day celebrating new works entering the US public domain). In some countries it is not even possible to legally waive (give up) one's copyright. And to make matters worse, copyright isn't the only possible restriction of an intellectual work, there are also patents, trademarks and other kinds of intellectual property.
Another bad news is that works in a "weak" public domain, i.e. most recent PD works or works that entered PD by some obscure little law, may as well stop being PD by introducing some shitty retroactive law (which has happened). So one may not be feeling completely safe going crazy by utilizing some recent PD works.
We therefore devise the term safe/strong public domain. Under this we include works that are pretty safely PD more or less world-wide, even considering possible changes in laws etc. Let us include these works:
Creative commons has created a public domain mark that helps mark and find works that should be in a world-wide public domain (this is not a waiver though, it is basically only used as a metadata for very old works to be better searchable).
There are a number of places on the internet to look for public domain works, for a list see below.
If you want to create a PD work (which you should), then generally in that work you must not use any non-public domain work. So, for example, you can NOT create a public domain fan fiction story about Harry Potter because Harry Potter and his universe is copyrighted. Similarly you can't just use randomly googled images in a game you created because the images are most likely copyrighted. Small and obscure exceptions (fonts, freedom of panorama, ...) to this may exist in laws but it's never good to rely on them, it's best to keep it safe and simply avoid utilizing anything non-PD within your works.
Also you can NOT use anything under fair use! Even though you could lawfully use someone else's copyrighted work under fair use, inclusion of such material would, by the fair use rules, limit what other would be able to do with your work, making it restricted and therefore not public domain. Example: you can probably write a noncommercial Harry Potter fan fiction and share it with friends on the internet because that's fair use, however this fan fiction can never be public domain because it can't e.g. be used commercially, that would no longer fall under fair use, i.e. there is a non-commercial-use-only restriction burdening your work. It doesn't even help if you get an explicit permission to use a copyrighted work in your work unless such permission grants all the right to everyone (not just your work). { I got a mascot removed from SuperTuxKart by this argument, mere author's permission to use his work isn't enough to make it free as in freedom. ~drummyfish }
So you can only use your own original creations and other public domain works within your PD work. Here you should highly prefer your own creations because that is legally the safest, no one can ever challenge your right to reuse your own creation, but there is a low but considerable chance that someone else's PD work isn't actually PD or will seize to be PD by some retroactive law change. So when it only takes a small effort to e.g. photograph your own textures for a game instead of using someone else's PD textures, choose to use your own.
{ NOTE: The above is kind of arguing for reinventing wheels which goes a little bit against our philosophy of remixing and information sharing, but we are forced to do this by the system. We are forced to reinvent wheels to ensure that users of our works can't be legally bullied. ~drummyfish }
In cases where you DO reuse other PD works, try to minimize their number and try to make sure they belong to the actual safe public domain (see above). This again minimizes legal risk and additionally makes it easy to document and prove the sources.
As a next step make sure you clearly document your work and the sources you use. This means you write down where all the works contained in your work come from, e.g. in your readme. Explicitly mention which things you have created yourself ("I, ..., have created everything myself except for X, Y and Z") and which things come from other people and where you have found them. It is great to also archive the proofs of the third party source being public domain (e.g. use the Internet Archive to snapshot the page with a PD texture you've found).
Finally you need to actually release your work into the public domain. It must be stressed that it is NOT enough to write "my work is public domain", this is simply legally insufficient (and in many countries you can't even put your work into public domain which is why you need a more sophisticated tool). You need to use a public domain waiver (similar to a license) which you just put alongside your work (e.g. into the LICENSE file), plus it is also good to explicitly write (e.g. in your readme) a sentence such as "I, ..., release this work into public domain under CC0 1.0 (link)". Currently the best waiver you can use is Creative Commons Zero (CC0) which is what we recommend. However note that CC0 only waives copyright and not other things like trademarks or patents, so e.g. for software you might need to add an extra waiver of these things as well.
NOTE: You may be thinking that it doesn't really matter if you waive your rights properly and very clearly if you know you simply won't sue anyone, you may think it's enough to just write "do whatever you want with my creation". But you have to remember others, and even you yourself, can't know if you won't change your mind in the future. A clear waiver is a guarantee you provide to others, not just a vague promise of someone on the internet, and this guarantee is very valuable, so valuable that whether someone uses your work or not will often come down to this. So waiving your rights properly may increase the popularity and reusability of your work almost as much as the quality of the work itself.
For an example of a project project properly released into public domain see the repository of our LRS game Anarch.
There are quite a few places on the Internet where you may find public domain works. But firstly let there be a warning: you always have to check the public domain status of works you find, it is extremely common for people on the Internet to not know what public domain is or how it works so you will find many false positives that are called public domain but are, in fact, not. This article should have given you a basic how-to on how to recognize and check public domain works. With this said, here is a list of some places to search (of course, this list will rot with time):
cat incategory:cc-zero. This site is quite reliable and serious about licensing, if you find a work marked as PD here, you can be reasonably sure this information is true.All content available under CC0 1.0 (public domain). Send comments and corrections to drummyfish at disroot dot org.