United States
American Philosophical Society http://www.amphilsoc.org/
Beinecke Library, Yale University http://www.library.yale.edu/beinecke/
Dumbarton Oaks Research Library and Collection http://www.doaks.org/
Firestone Library, Princeton University (NJ) http://library.princeton.edu/
Free Library of Philadelphia http://www.freelibrary.org/
John Carter Brown Library http://www.brown.edu/Facilities/John_Carter_Brown_Library/
Houghton Library (Harvard) http://hcl.harvard.edu/libraries/houghton/
Huntington Library, San Marino
Lewis Walpole Library Yale University (Farmington, CT) http://www.library.yale.edu/walpole/
Library Company of Philadelphia http://www.librarycompany.org/
The Metropolitan Museum, New York
The National Archives http://www.archives.gov/
The National Gallery of Art, Washington DC
National Humanities Center http://nationalhumanitiescenter.org/
New York Public Library http://www.nypl.org/
William Andrews Clark Memorial Library UCLA http://www.humnet.ucla.edu/humnet/clarklib/
Winterthur Museum, Garden and Library (Delaware) http://www.winterthur.org/
China
Arthur M. Sackler Gallery and Freer Gallery of Art
National Palace Musuem (Taiwan)
Number One National Archives (Beijing)
The Metropolitan Museum, New York
France
Archives nationales http://www.archivesnationales.culture.gouv.fr/
ARTFL project http://artfl-project.uchicago.edu/
Bibliotheque Nationale de France
Réunion des musées nationaux http://www.photo.rmn.fr/cf/htm/Home.aspx
Germany
Göttingen State and University Library (among the most complete and searchable sites in Europe)
Great Britain
The Cambridge University Library
India
British Library, London www.bl.uk
National Archives of India, New Delhi http://nationalarchives.nic.in/
Royal Asiatic Society, London www.royalasiaticsociety.org
Italy
American Academy in Rome http://www.aarome.org/
Biblioteca Ambrosiana, Milan http://www.ambrosiana.eu/jsp/index.jsp
Bibliotheca Hertziana, Rome http://www.biblhertz.it/english/home/default.htm
French Academy in Rome http://www.villamedici.it/
Villa I Tatti: The Harvard University Center for Italian Renaissance Studies http://itatti.harvard.edu/
Japan
Tokugawa Art Museum http://www.tokugawa-art-museum.jp/english/
Edo-Tokyo Museum http://www.edo-tokyo-museum.or.jp/english/
Ukiyoe Archive http://www.ne.jp/asahi/kato/yoshio/index.html
Waseda University Library http://wine.wul.waseda.ac.jp/
Netherlands/ Belgium
CODART: Dutch and Flemish art in museums worldwide www.codart.nl/
RKD Netherlands Institute for Art History http://english.rkd.nl/Latest%20News
Rijksmuseum Research library http://library.rijksmuseum.nl/cgi-bin/koha/opac-main.pl
Royal Museum for Fine Arts Antwerp (Koninklijk Museum voor Schone Kunsten Antwerpen) http://www.kmska.be/en/
Mauritshuis (museum), The Hague http://www.mauritshuis.nl/index.aspx?siteid=54
Rijksmuseum Amsterdam http://www.rijksmuseum.nl/
Spain/ Americas
Archives-LANIC (links) http://lanic.utexas.edu/la/region/archives/
The Bancroft Library, University of California-Berkeley http://bancroft.berkeley.edu/
Benson Latin American Collection, University of Texas at Austin http://www.lib.utexas.edu/benson/
Latin American Library, University of Florida http://www.uflib.ufl.edu/lac/
Latin American Libraries-LANIC (links) http://lanic.utexas.edu/la/region/library/
The Latin American Library, Tulane University http://lal.tulane.edu/
The Lilly Library, Indiana University http://www.indiana.edu/~liblilly/index.php