Public Domain
Public domain refers to the concept that intellectual property is not controlled by any person or entity. Works usually fall into this category because the copyright of the work has expired, but there are other ways materials can fall into this category. Intellectual property listed as public domain may be used freely, though there may need to be a citation.

The Public Domain (Stanford University Libraries)
A brief overview by Stanford on public domain covering the various ways works get into the public domain and the potential pitfalls like multilayered works, works protected by another country, works protected by trademark law, etc.

Hirtle’s Copyright and the Public Domain
A concise easy to read chart developed by Peter B. Hirtle that lists copyright terms of various works with the dates these works will be available in the public domain.

The Copyright Genie
Not sure if your work is public domain? This interactive tool developed by Michael Brewer & the ALA Office for Information Technology Policy helps answer the question – is it covered by copyright and by what terms.

Public Domain Sherpa Calculator
Another interactive tool to help users keep track of which U.S. copyright act applies to work.

The Online Books Page – How Can I Tell Whether a Book Can Go Online?
This page created by the University of Pennsylvania walks a user through the steps to determine whether a book can go online. In their public domain section, UPenn provides information on copyright duration in various countries as well as links to more detailed copyrights laws in those nations. Good resource if one needs to look into non-US copyright law.

The Public Domain Review
A not-for-profit project by the Open Knowledge Foundation highlighting unusual and interesting works that are available in the public domain.

Where to find public domain works?
University of Montana – Public Domain and Creative Commons: A Guide to Works You Can Use Freely
This guide contains a surplus of links to resources where one can find public domain works sorted by medium (written, music, visual arts/photographs, maps).

Smithsonian Institution Public Domain Images
Smithsonian’s public domain image collection

New York Times Public Domain Archive
Images from the NYT in the public domain

Project Gutenberg
Public domain electronic books

Public domain audio books

*Image found on Creative Commons site.