Reference Sources - scope out an area, provide brief, better bibliography and key words (author's names, nomenclature) for more extensive searching.
Groveart (Oxford Online) - Scholarly art encyclopedia. Entires on classical, also use for terms and other art topics related (Asia Minor, etc.)
Brill's New Pauly - combined classical-subject reference works, the most important of which is the Pauly (English).
Oxford Reference Online: Classics - Oxford University Press online titles on classical studies
Lexis - full-text newspaper index going back +10 years. Great for seeing classical context in modern times
Dukecatalog example, for women in the classical world, use the subject heading, Women--Greece
Perseus Project - Classical studies website maintained by Tufts University. Compendium of texts and images.
Claros - The World of Art on the Semantic Web - Oxford University's classical art website (images)
AWOL - Ancient World On Line - serious blog for the study of the Ancient world.