This selective lists includes sources for non-fiction, fiction, poetry, and drama.

Eldritch Press
http://eldritchpress.org/

While this may not be the longest list of public domain materials, it does contain American literature, English literature, and literature in English translation from across the world.

Google Books
http://books.google.com/advanced_book_search

Not all material listed here is in the public domain, but you can search for books and magazines published prior to 1923 by using the Google Books Advanced Search. 

Government Documents
http://www.gpoaccess.gov/index.html
http://www.gpo.gov/

Find works created by Federal Government agencies. 

Internet Archive: Text Archive
http://www.archive.org/details/texts

Some of the texts on this site also contain Creative Commons licensed works.  Contains quite a bit of content. 

Library of Congress
http://www.loc.gov

In addition to other online and print collections of public domain works, also be sure to check out the Library of Congress's Digital Collections which can provide access to public domain materials over the Internet on a variety of topics.  Please also be sure to double check that the material you use is actually in the public domain.  The introductory web pages for each collection typically contains a link to a copyright page-- here is an example from the National American Woman Suffrage Collection.    

Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page

Be sure to have a look at Project Gutenberg's License page, which explicitly makes a distinction between using the raw public domain content in its e-books and Project Gutenberg's trademarked and licensed content.  You can read this information-- in layman's terms-- under the heading "Public Domain Books." 

Wikipedia
http://en.wikipedia.org/wiki/Wikipedia:Public_domain_resources

This site consists of a list of links to other Internet sites which provide access to public domain works.  I recommend this site with a bit of caution in that you should verify the public domain status of any materials you locate.    

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.

American Memory Collection (Motion Picture and Television Reading Room, Library of Congress)
http://www.loc.gov/rr/mopic/ndlmps.html

Films included in this collection are mostly in the public domain, however, please verify the status before using a film.

Sites selected for this list mostly provide access to public domain sheet music. 

Historic American Sheet Music (Duke University)
http://library.duke.edu/digitalcollections/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
http://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
http://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)
http://memory.loc.gov/ammem/smhtml/smhome.html

Sheet music collection dating from 1870-1885.  Most of the content should be in the public domain, but you need to verify it. 

Musopen
http://www.musopen.com/

Another site providing access to public domain music.  Does not contain as much content as IMSLP.

Mutopia Project

http://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

http://www.pdinfo.com/about.php

This site is more proprietary in nature, but provides a good list of musical works that are in the public domain.

Sites selected for this list were included because the majority of the links provide access to public domain map content.

"Maps Library Guide" by Jennie Burroughs and Donna McCrea of the Mansfield Library
http://libguides.lib.umt.edu/maps

Contains maps by topic: historic, scientific, topographic, Montana maps, etc.  Ones created by U.S. Government Agencies by U.S. Government employees are in the public domain.  You need to verify the copyright status of other maps you find by using this guide. 

Perry-Castañeda Library Map Collection-- University of Texas Online
http://www.lib.utexas.edu/maps/

Includes maps from all over the world.  According to their frequently asked questions many of their maps are in the public domain.  The ones that are not are clearly marked as being protected under copyright law. 

"Sources of Public Domain Maps" Public Domain Sherpa
http://www.publicdomainsherpa.com/public-domain-maps-resources.html

Another recommended link from the Public Domain Sherpa site.  Site provides many more links to sites where you can access public domain maps.