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CC0
CC0 is a waiver (similar to a license) of copyright, created by Creative Commons, that can be used to dedicate one's work to the public domain (kind of).
UPDATE: There is now a similar waiver called WPDD (worldwide public domain dedication, https://wpdd.info/), intended to also waive patents.
Unlike a license, a waiver such as this removes (at least effectively) the author's copyright; by using CC0 the author willingly gives up his own copyright so that the work will no longer be owned by anyone (while a license preserves the author's copyright while granting some rights to other people). It's therefore the most free and permissive option for releasing intellectual works. CC0 is designed in a pretty sophisticated way, it also waives "neighboring rights" (e.g. moral rights; waving these rights is why we prefer CC0 over other waivers such as unlicense), and also contains a fallback license in case waiving copyright isn't possible in a certain country. For this CC0 is one of the best ways, if not the best, of truly and completely dedicating works to public domain world-wide (well, at least in terms of copyright). In this world of extremely fucked up intellectual property laws it is not enough to state "my work is public domain" -- you need to use something like CC0 to achieve legally valid public domain status.
WATCH OUT: don't confuse CC0 with Creative Commons Public Domain Mark (apart from name the symbols are also a bit similar), the latter is not a license or waiver, just a tag, i.e. CC0 is used to release something to the public domain, while PD mark is used to mark that something is already in the public domain (mostly due to being old).
CC0 is recommended by LRS for both programs and other art -- however for programs additional waivers of patents should be added as CC0 doesn't deal with patents. CC0 is endorsed by the FSF but not OSI (who rejected it because it explicitly states that trademarks and patents are NOT waived).
It's nice that CC0 became quite widely used and you can find a lot of material under this waiver, but BEWARE, if you find something under CC0, do verify it's actually valid, normies often don't know what CC0 means and happily post derivative works of proprietary stuff under CC0.
Some things under CC0 include Librivox audiobooks, Dusk OS, Wikidata database, Lichess chess database, great many things on sites like Wikimedia Commons, opengameart (see e.g. Kenney), Blendswap, freesound etc., whole Esolang Wiki, OSdev Wiki (since 2011), Encyclopedia Dramatica (EDIT: seems like they dropped it now :D Internet archive has the old CC0 version still), LRS software (Anarch, small3dlib, raycastlib, SAF, comun) and LRS wiki, books like The Pig and the Box (anti DRM child story) or Cost of Freedom, some fonts by dotcolon, Lix (libre game), evlisp minimalist Lisp (from book "Lisp From Nothing") and many others.