University of St. Andrews (UK)
University of St. Andrews: Albrecht Durer
An excellent, detailed biography of Durer, with extensive links to his works and contemporaries. This site not only details his artwork, but also his contributions as a mathematician.
Web Gallery of Art
Web Gallery of Art: Masaccio
This site from the Web Gallery of Art provides a biographical sketch on Masaccio, "the first great painter of the Italian Renaissance." Includes link to related information and to examples of Massacio's artwork.