The Smithsonian Institution Libraries' Galaxy of Images contains a sampling of digitized materials from the many collections housed at the Smithsonian. In addition to the many digital collections created and maintained by library staff, the Galaxy of Images makes available high-resolution scans of materials that support research from the Smithsonian's holdings. Users can search or browse the images.
Some of my favorites:
Hand-tinted lithographs of "life on the moon"
Lily plant illustration from 1500
Workingman's Model Home, 1893
Chocolate bunny playing a guitar