It's REALLY awesome to see the Smithsonian do this, and I can't wait to see how it fuels artists going forward. Lots of opportunities to use this content in a variety of ways.

Along with the Smithsonian, there's a bunch of other galleries and museums doing this as well:

NYPL Digital Collection

The Met

Rijks Museum

Art Institute of Chicago

In some cases you might need to select "public domain" on the searches.