These sites provide collections and reference materials that can supplement textbooks and other course materials.
Open Learn - History & the Arts -- Open Learn is an initiative of The Open University, UK. It provides supplemental materials for educators, including articles, podcasts, videos, games, and much more. In addition, it offers more than 700 free online classes.
The Met Curriculum Resources -- Introduce your students to the richness and diversity of works of art in selected collections of the Museum. These comprehensive and fully illustrated guides provide useful background information and ideas for engaging students.