NYPL Digital Library

From top to bottom: Thomas Nast, John Singer Sargent, Henrik Ibsen

One of the things I miss most about moving from New York (although I'm only an hour commute into Mahattan) is the museums, galleries, and the main branch of the New York Public Library.
The NYPL has been archiving a digital gallery that provides free and open online access to thousands of images from the original and rare holdings in it's possesion. Spanning a wide range of visual media, NYPL Digital Gallery offers digital images of drawings, illuminated manuscripts, maps, photographs, posters, prints, rare illustrated books, and more. The site is a great reference source and worthy of bookmarking.