Getty Provenance Index - Ongoing list of paintings and who owned them, beginning from 1801.
International Foundation for Art Research - authoritative information on authenticity, ownership, theft, and other artistic, legal, and ethical issues
International Culture Property Protection - U.S. State Department's webpage, includes imagesconcerning art objects.
Lost Art Internet Database - a service of the Koordinierungsstelle für Kulturgutverluste, Germany’s central office for the documentation of lost cultural property, Germany.
Archives of American Art - Federal collection of primary source material pertaining to art. Good for art collectors, art dealers and art historian's letters, and other documentation.
Art Provenance Map - Open-Source mapping tool of the provenance trail for individual paintings (very cool!).
World War II
The Document Project for Wartime Cultural Losses - Loyala University's database of objects.
United States National Archives Search - use the boxes, below right, to search the Archival Research Catalog, EVetRecs, etc. for official documents created in connection with works of art either recovered or documented.
Looted Art - Central Registry for Information on Looted Cultural Property, 1933-1945
(many museums run pages such as these)
Metropolitan Museum of Art Provenance Research Project - Website of one of the leaders in provenance research, especially its link to provenace/thefts sites.
Arcade catalog - online search of the combined catalogs of the Frick Art Reference, Brooklyn Museum and Museum of Modern Art libraries on the topic of "WW II provenance research"
Carnegie Mellon Art Tracks - structured data for provenance using a suite of open source software tools
Names of collectors, scholars, dealers and purloiners can be traced using these sources:
Lexis/nexis Academic - articles in major U.S. newspapers. By switching defaults to "World Newspapers", you can search major European dailies as well.
New York Times (1851-3 years ago) - the pdf version of all articles in the NYT. Good for obituaties and articles on disputed estates and objects of art.
Times Digital Archive - like above, only for the Times, London.
Comprehensive Art Search (Art & Architecture Complete, Art Index Retrospective, Art Abstracts) - the major journal article source for art; 1929-present. A suite of databases including Art Full Text, Art Index Retrospective, Avery Index (architecture), Cinema Gallery (stills from films and movie posters) and Art Museum Image Gallery. This link works only for Duke IP-adress authenticated computers. For off-site validation users, use Art & Architecture Complete.
Bibliography of the History of Art (BHA) - though limited to articles from the 1950's forward, a fairly comprehensive search of not only journal articles, but conference proceedings and catalog essays.
Jstor - make sure to limit to "Art History," "Classical Studies," "Archaeology," "Architecture," and possibly "History" depending upon your research.