Arts Edge: Not so much information about materials but a wonderful site for art educators to explore.
ArtLex: Art dictionary for artists, collectors, students and educators in art production, criticism, history, aesthetics, and education.
Conservation & Art Material Encyclopedia Online (CAMEO) by the Museum of Fine Arts Boston.
Pigment Information. Historical, chemical, and artistic importance of various pigment used by artists.
Wilhelm Imaging Research: Information about the permanency of inkjet printing.