http://photography.si.edu/ _'invites you to explore the world of Smithsonian photography'_ by creating sequences of user-tagged images and sharing them with others. Of the hundreds of available photographs to manipulate, order, and play with, I couldn't find one that wasn't absolutely beautiful. This site is worth visiting just for the great pictures, but the very interactive way in which the photos are presented makes it even more fun, transforming a traditionally passively-enjoyed art form into something more communicative.