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).
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" (moral rights), 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.
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).
Here are some things and places with CC0 materials that you can use in your projects so that you can release them under CC0 as well. 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.
All content available under CC0 1.0 (public domain). Send comments and corrections to drummyfish at disroot dot org.