Artists Bluebook, American Artist Reference: Free information on 23,000 American artists including biographies, bodies of work, valuation and appraisal techniques, auction records, publications, and artists representatives as well as literature and museum information for American painters, sculptors, and artists of all mediums, subjects, and genres.
Art History Research Center: The AHRC is the first and foremost tool for art historical research on the internet.
Need to find a picture to use with your paper or presentation? The Art Archive is a good starting place.