Public domain materials are also free to use and reuse because they are not registered as copyrighted, or their copyrights have expired. For an overview of public domain, see this FAQ from TeachingCopyright.org.
The following links are some good resources for finding public domain works:
The Smithsonian's flickr account of images in the public domain that they have digitized from their collections.
What is Open Access?
Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions. OA removes price barriers (subscriptions, licensing fees, pay-per-view fees) and permission barriers (most copyright and licensing restrictions). The PLoS shorthand definition —"free availability and unrestricted use"— succinctly captures both elements.
To find open access resources, check out the list of OA databases in this guide. If you have a resource that you want to share with someone who doesn't have institutional access, here are a few databases you can use to see if an open access version of the resource is available online: