There comes a point when copyright and free licensing aren’t right; when the original creator’s right to own his/her artistic work becomes less important than the right of every person to have access to and use that work. At this point, the work becomes public domain: something that is the property of anyone and everyone.
There are some things that belong to everyone: the air we breathe, sunlight, etc. Similarly, there are public spaces – village greens, town squares, etc. – that are for the use of everyone. Similarly, there is an area of intellectual creativity that is available to everyone: the public domain. These works are for everyone to use, share, re-use, remix, etc.
After an artist or creator dies and the copyright on his/her work expires, that work passes into the public domain. The plays of Shakespeare, the symphonies of Beethoven, the paintings of Leonardo da Vinci, etc, are all in the public domain: nobody owns the rights to them, so that anyone and everyone can use them.
There is some difference and disagreement as to exactly when a work protected by copyright laws passes into the public domain. Most countries rule that either 50 or 70 years after the death of the creator, copyright protection expires. The United States has changed its laws several times, so the determination of when a work enters the public domain in the US is very complicated. See this explanation.
Derivative works based on works in the public domain can be protected by copyright. So if I create an artwork based on the Mona Lisa but incorporating my own personal touches, I can copyright that work. Similarly, while the music of Beethoven or Mozart has entered the public domain, any performances of their works can be protected by copyright laws.
Anyone can voluntarily give up their rights to copyright and donate their work to the public domain. Creative Commons has created a CC0 license that creators can apply to their work which gives up all rights. This allows people to use their work in any way they wish …even without attribution.
There are many places to find works in the public domain which you can use, enjoy, share, and incorporate into your own creative works. Here are a few places to start:
Europeana: a collection celebrating and promoting the cultural works of Europe. (Note: not everything on the site is necessarily public domain, but the site is committed to public access to culture, and much of the site is available under open licenses.
NASA: all the amazing photographs and videos taken by NASA spacecraft and astronauts are automatically entered into the public domain. All works created by US government employees in the course of their work become public domain because they are paid for by (US) public funds. NASA is just one example.
Pixabay: a curated photography site (editors check submissions for quality) with all submitted photographs (or vector graphics) licensed CC0. (I have submitted photographs here – and some weren’t selected. Photographs selected by this site are generally of high quality.)
Project Gutenberg: fill up your e-reader for free: Project Gutenberg has tens of thousands of free e-books that are in the public domain.
Musopen: a music site that has public domain performances – recordings that are free to download, listen to, remix, sample, etc. The site also offers public domain sheet music, and has sponsored recordings to be entered into the public domain. For example, the site commissioned recordings of the complete works of Frederic Chopin.
Public Domain Day: less of a source of public domain works, and more a commentary on the state of public domain in the US. This site celebrates works that are released into the public domain every year on January 1st. Or at least, released into the public domain in Canada or Europe. As the site notes, nothing will be released into the public domain in the US until 2019!
Public Domain Sherpa: a site devoted to giving information and help in finding works in the public domain in the US.
Public Domain Movies: there are various sites that host movies that are in the public domain. The Internet Archive is a great source of classic copyright-free movies, as well as other copyright-free media.
I've been teaching and traveling the world for decades. I teach technology skills and programming in international schools, and love developing skills in my students. Teaching internationally gives me a broader perspective and I thoroughly enjoy the thrill of new sights and experiences.