Smarthistory Books - Published by the scholars behind Smarthistory/Khan Academy, these volumes feature essays on specific artworks in various categories, from Asian to contemporary to antiquity.
Pressbooks - Featuring over 6,500 textbooks and books, this site is where many academic institutions are publishing OERs, including Johns Hopkins University, University of Massachusetts, Amherst, and more.
Smarthistory/Khan Academy - This site features videos, images, and articles written by academics, on art from antiquity until now.
Heilbrunn Timeline of Art History - From the Metropolitan Museum of Art, this site features detailed essays written by curators, accompanied by images of works from the museum's collection.
Teaching Commons - This site divides its content into textbooks, course sites, syllabi, course materials, and lectures across all disciplines.
edX - This massive open online course (MOOC) founded by Harvard and MIT provides classes in a range of subjects.
Coursera - This site features constantly rotating courses from universities around the world covering virtually every topic, all of which include video lectures, assignments, and discussion forums.