Art History 

Art journal databases are specialized - they're by authorities ("peer reviewed") and have special ways to get at art topics.

Art Abstracts/Art Index - the major journal article source for art.  Select either of the two databases and click the other (upper left selection) in order to search comprehensively from 1929

Artbibliographices Modern - just 19th-21st-century art.  Indexes down to essays in exhibition catalogs.  Multi-lingual.

Bibliography of the History of Art - Broadest coverage, before 1970's strong French coverage

Jstor - limit to "Art History" in area content

Architecture

Avery Index - premier index for architecture journals, run by Columbia University.

Asia

Bibliography of Asian Studies - Articles in Western languages regarding aspects of Asian and South Asian studies, including art history and popular culture.

Auction Catalogs

Scipio - useful for tracking down individual works of art, especially provenance

Medieval

ITER - Indexes journals covering European Renaissance (1300-1700) and medieval eras (400-1500).

Newspapers - newspapers are very important for reading exhibition reviews, tracing media coverage of art or artists, popular reception to art and tracing works of art.

Lexis/Nexis Academic - newspaper articles from the late 1980s onward (depending on the newspaper).  Separate search for European dailies is also excellent.

America's Newspapers - search American newspapers by region of the country or individual newspaper.  Great for seeing how an art show was reviewed in rural America.

Frequently historic newspaper accounts can be primary sources for exhibitions and other reception documentation.  Also good for patronage verification and biographical/obituary accounts.  Both pdf scans of the actual column.

Times Digital Archive - the Times (London), 1785-1985

New York Times Digital Index - 1851 until three years ago.