Public domain (or PD for us open content geeks) is the absence of any copyright restrictions and licensing requirements – public domain content gives you absolute freedom in how you use it. This is important in, say, a wiki, where public domain content can be used as the basis of an article – as was done for many articles in Wikipedia, using old, out-of-copyright encyclopedia articles.
The Public Domain Review has published a Guide to Finding Interesting Public Domain Works Online, a great guide to finding public domain cultural works, in particular. But they’ve missed my favorites – the scattered works of the US federal government.
Appropedia is about science and technology – not necessarily the newest technology, but the most appropriate technologies and methods in construction, energy, water, sanitation, agriculture and other areas related to sustainable living. Guides and manuals, best practice, reports, impact studies, analysis – this kind of content is often found in governmental and intergovernmental publications, and while most governments’ works are copyrighted, in a few cases it is open content.
In particular, work created by officers of the U.S. federal government is generally public domain, by law. However, it’s not enough to searching in the*.gov domain, as that includes vast amounts of state and local government material which is not public domain, or even open-licensed. These pages also don’t use anything like the Creative Commons “mark” which helps search engines identify pages by license.
For that reason I’ve put together a custom search engine for the public domain – mainly searching the .gov domain while excluding a long list of non-PD .gov sites (more than 400 so far, most of them identified manually). It needs more work, possibly by an IP intern, identifying and excluding non-PD sites, and the onus is on the user to check the status of the material, but if you’re after public domain material of a serious nature, try it out.