View the Guggenheim database of virtual collections containing over 700 artworks by nearly 300 artists. "Founded on a collection of early modern masterpieces, the Guggenheim Museum today is an ever-growing institution devoted to the art of the 20th century and beyond."
"NewLits.org is a wiki space created to collaboratively develop a rich range of specialist resources for middle school language arts/literacy educators (typically Grades 5 to 8). These resources focus variously and broadly on new literacies and digital technologies.
The Louvre has an excellent digital library. Explore Islamic art, Roman antiquities, and works by the masters. Be sure to explore the link for teachers.