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: