ADAM
Gateway to art, design, architecture and media information on the Internet. Searchable database selected by professional art librarians.
http://adam.ac.uk/

Artcyclopedia
Guide to museum-quality art on the Internet. Search hundreds of art museum sites for exhibits and artists.
http://artcyclopedia.com/

Artnet .com
An arts portal, with artist listing, galleries, and auctions.
http://www.artnet.com/