The Metropolitan Museum of Art has put images of public domain artworks under the Creative Commons Zero (CC0) License.
Here's an excerpt from the announcement:
This morning, we announced a major update to the Museum's policy governing the use and reuse of images in our collection: all images of public-domain artworks in the Museum's collection are now available for free and unrestricted use under Creative Commons Zero (CC0). This updated policy, known as Open Access, enables everyone to utilize more than 375,000 images of public-domain artworks in The Met's collection in any media without permission or fee.
See also: "Introducing Open Access at The Met."