Contemporary and Digital
Museum of Contemporary Art, Chicago
Museum of Contemporary Art, Los Angeles
Beinart collection of Surrealist Art Museum of Digital Art - some very good.
The Museum of Modern Art, NYC San Francisco Museum of Modern Art
United States
National Gallery of Art - great online exhibits including Cezanne in Provence
Art Institute of Chicago's Art Explorer
The Metropolitan Museum of Art
The Museum of Fine Arts, Boston Albright-Knox Art Gallery
Fine Arts Museums of San Francisco
Carnegie Museum of Art, Pittsburgh
International
Kunsthistorisches Museum, Vienna
National Galleries of Scotland
Finnish National Gallery, Helsinki
German National Museum, Berlin
Kawamura Memorial Museum of Art, Japan