Sites selected for this list mostly provide access to public domain sheet music.
Historic American Sheet Music (Duke University)
https://repository.duke.edu/dc/hasm
Most of the materials here are in the public domain, but please check the dates or other features to be certain.
International Music Score Library
https://imslp.org/
Well-organized wiki-based site for accessing public domain and a few Creative Commons musical works.
Lester S. Levy Collection of Sheet Music
https://levysheetmusic.mse.jhu.edu/
Contains popular American sheet music from 1780 through 1980. Just from looking at the range of dates, not everything is in the public domain; be sure to check dates and other features.
Music for the Nation: American Sheet Music (Library of Congress)
https://www.loc.gov/collections/american-sheet-music-1870-to-1885/about-this-collection/
Sheet music collection dating from 1870-1885. Most of the content should be in the public domain, but you need to verify it.
Musopen
https://musopen.org/
Another site providing access to public domain music. Does not contain as much content as IMSLP.
Mutopia Project
https://www.mutopiaproject.org/
Some music available here may be covered by Creative Commons licenses; others are in the public domain. Please verify how a piece of music can be used.
Public Domain Information Project
https://www.pdinfo.com/index.php
This site is more proprietary in nature, but provides a good list of musical works that are in the public domain.
This selective list contains links that can provide you with access to art collections (original art objects that may be in the public domain) as well as art reproductions that could be in the public domain. While it can be fairly easy to locate actual works of art in the public domain, reproductions of public domain art works may not be in the public domain. As always, please verify the copyright status of whatever material you'd like to use for your project.
Locating Original art works (Sculpture, drawing, painting, photography, etc.):
This list will help you to locate galleries, museums, or other collections that could contain public domain original art works. Visiting web sites of art museums and galleries can be very helpful to locating the actual public domain art object you seek.
"Free Art Resources on the Web" Art Libraries Society of North America
http://www.arlisna.org/pubs/onlinepubs/freeart.html
This resource contains links to trusted art-related web sites used by art librarians.
Yahoo Directory & Art Museums, Galleries, and Centers
http://dir.yahoo.com/Arts/Museums__Galleries__and_Centers/Web_Directories/?b=0
Some canned Google searches that may prove helpful:
1. Art museums and galleries under the domain .org
2. Art museums and galleries under the domain .edu
Locating Art reproductions, photographs, other images in the Public Domain:
This list will help lead you to public domain art reproductions, photos, and other images. Again, it's important to remember that even though an art object might be in the public domain, its reproduction may not be. Here are a couple of resources to consult which further explain this potential problem area when using reproductions of public domain visual works.
"Finding Images" Art LibGuide (Mansfield Library)
I put together this guide initially to help users locate a variety of images. Some of these links may take you to public domain image reproductions.
Copyright-Free Photo Archive: Public Domain Photos and Images (gimp-savvy.com)
This is a site that provides access to images created by U.S. Government entities such as NOAA, FWS, and NASA.
"Finding and Using Public Domain Photographs" Public Domain Sherpa
This page is from an informative public domain site written by an independent entrepreneur/attorney. Author provides a lengthy list of sites that provide access to public domain photos because the original work's copyright duration expired or the original work was created by a U.S. Government agency.
U.S. Government's Official Web Search (for images)
Use this site to help you search for images created by the U.S. Government.