Infinite Photograph is a photo mosaic of more than 300,000 nature photographs that just keeps going and going and going. For example, the image on the left it is a photo mosaic of a chickadee head, which you can see is made up of hundreds of tiny photos. Zoom in (with the yellow square in the bottom corner) to see the other photos, and all of those photos are mosaics as well. It is really quite astounding.
If you enjoy this you may also like NASA's Visible Earth, the Smithsonian's Earth from Space, and the always wonderful The Big Picture.