The Library of Congress (with the help of Merrill Lynch) has put up The Floating World of Ukiyo-E (Library of Congress Exhibition), which collects three centuries of Japanese woodblock prints. There's a couple of Hiroshige and Hokusai prints, but the really interesting ones I thought were the surreal :

There's also this print of frogs dressed up as Kabuki actors dressed up as samurai.

Looking at these prints it's easy to see understand the aesthetic used in many anime (like Spirited Away).

(via Making Light)