Another awesome digital archive:  The Getty Museum/Foundation/Research and Conservation Institute has a search gateway were you can find some fabulous images, plenty of which are within the public domain.  Check out GSG: highlights=Open Content Images.

I am fascinated by Daguerreotype and silver prints.  Did you know that Daguerreotypes captured the weave of the cloth and the stitching on the clothes of people who sat for portraits?  That is just fabulous!  It is endlessly enlargeable, what modern camera can say that?