Once you activate your UNI (University Network ID), you can access resources licensed by the Columbia University Libraries that are not openly accessible online.  Among these licensed resources are indexes such as Art Index, Avery Index, Bibliography of the History of Art, encyclopedias such as Grove Dictionary of Art, Encyclopedia Britannica Online, full text resources such as LEXIS/NEXIS, Proquest Historical Newspapers, JSTOR, Project Muse, image collections such as ArtStor, AccuNet/AP Multimedia Archive and thousands of other licensed online resources for which we pay a considerable sum per year. Please understand that you will not be able to access most of these materials if you try to get to them through an open web search (e.g. Google, Yahoo, etc.).

On the other hand, here are several extensive, freely accessible online resources for Art History on the Web:

Art history resources on the web (Online)
Presents a collection of art history resources, compiled by Chris Witcombe. Includes the following periods/areas: Prehistoric, ancient Near East, Egyptian, Greek, Roman, late antique, early Europe, Islamic, early Medieval, Gothic, Renaissance, Baroque, 18th century, 19th century, 20th century, contemporary, prints & photographs, Chinese, Japanese, Indian, Southeast Asian, African, the Americas, Pacific cultures, research resources, miscellaneous, museums & galleries.

Arthistoricum.net (Online)
Art and art historical portal realized thanks to financial support from the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG), as a common project of the Library of the Zentralinstitut für Kunstgeschichte in Munich and Heidelberg University Library in co-operation with the Institute for Art History of the LMU Munich (chair Professor Dr. Hubertus Kohle) together with the historical network historicum.net and further partners. arthistoricum.net focuses the core competencies of the co-operating institutions into a fundamental platform for research and teaching.   It includes the following sections: Electronic Journals Library, ARTicles online-Art historical articles from e-journals, artlibraries.net (listed below), Art historical institutes, etc.

CODART
An International Council for curators of Dutch and Flemish Art. This site is self-described as the best guide to Dutch and Flemish art in museums worldwide.