(these are the sites that I would give awards to)
Probably the best and most complete art site on the web, with a complete auction database, substantial gallery compendium, up-to-date art magazine, and up and coming chat room.
The Philadelphia Museum of Art's experimental virtual reality gallery. The inaugural project is "Brancusi's Mademoiselle Pogany." This exhibition exists only on the web in three-dimensional virtual reality, allowing you to "walk" around the sculpture, and to explore the development of an important group of related works.
Check out the Diego Rivera Archives documenting the creation of the DIA's Rivera mural, showing before and after pictures, details of the mural superimposed on Rivera's own photographs of the Ford assembly line, as well as mural details superimposed over Rivera's preparatory sketches. This is an adjunct to their substantial art database.