Invaluable is an auction website, containing a database of artists and auctions. They are providing access to their Professional level subscription with 50 million auction entries and information on more than 500,000 artists. Note: be aware that, although you will not be charged, the registration process for Invaluable currently requires that you provide billing details.
Oxford Art Online offers access to Oxford's art reference works: Grove Art Online and the Benezit Dictionary of Artists, as well as many specially commissioned articles and bibliographies available exclusively online. Oxford Art Online has over 20,000 images and over 230,000 entries on art and artists.