The New York Public Library has released its public domain archives online in an experimental graphic format that makes searching for images fun. The interface lets you survey thousands of images at a time in a photo mosaic of tiny thumbnails (above).
On their site, when you scroll over the little tiles, they pop up into a larger image. Double clicking opens up access to the image in various scales of resolution, most of which you can access for free. They're even set up to take orders for printed wall posters.
I found this 19th century photograph of a clown taken by the photography studio Jeremiah Gurney & Son.
You can select a category by century, genre, collection, or color, and then drill down to specific images that might interest you. There are stereoscopic views, old postcards, manuscripts, atlases, menus, portraits, prints, and drawings.
The public domain remix was developed by Brian Foo of NYPL Labs, using the data released by NYPL.