A wealth of free art resources can be found online, and I’d like to call attention to two of them.
The Metropolitan Museum of Art has launched MetPublications, which will “eventually offer access to nearly all books, Bulletins, and Journals” published by the Met since 1870. Now available are well over 300 catalogs and other publications, which can be downloaded as pdf files or viewed online. Both options involve a little patience: long download times in the case of the former, and a lot of zooming in for the latter. But the riches that can be found there are considerable. You should check out the entire list yourself. Just scanning the letter A, and obviously reflecting some of my personal interests, I found:
Abbot Suger and Saint-Denis
Al-Andalus: The Art of Islamic Spain
Along the Riverbank: Chinese Paintings from the C. C. Wang Family Collection
American Paradise: The World of the Hudson River School
Ancient Art in Miniature: Ancient Near Eastern Seals from the Collection of Martin and Sarah Cherkasky
Art and Autoradiography: Insights into the Genesis of Paintings by Rembrandt, Van Dyck, and Vermeer
The Art of Illumination: The Limbourg Brothers and the Belles Heures of Jean de France, Duc de Berry
Art of Island Southeast Asia: The Fred and Rita Richman Collection
The Art of Precolumbian Gold: The Jan Mitchell Collection
Art of the Bronze Age: Southeastern Iran, Western Central Asia, and the Indus Valley
Art of the Dogon: Selections from the Lester Wunderman Collection
This comes just months after the Guggenheim Museum made almost seventy modern art catalogs available online. Here are a few from that collection that particularly caught my attention:
Art of the Avant-Garde in Russia
Francis Bacon
Alexander Calder: A Retrospective Exhibition
China: 5,000 Years – Innovation and Transformation in the Arts
Jean Dubuffet: A Retrospective
On the Spiritual in Art by Wassily Kandinsky
Paul Klee 1879-1940: A Retrospective Exhibition
Gustav Klimt and Egon Schiele
Edvard Munch
Mark Rothko, 1903-1970: A Retrospective
Thirty-Two Minutes 
This is interesting; I didn’t realize they put full catalogues free online. Seems a great resource to me. thanks I just looked at the Munch one. Might check out the Rothko one too.