When I was kid, my brother bought a book that contains 3D illusion of Casper. The picture is like an abstract image but if you focus closely to the picture, suddenly the picture will become blur and then an image suddenly pop out of the picture, revealing the 3D illusion image.
If you see the abstract picture above using the correct technique, you should see a Saturn planet pop out from the picture. I still wonder if there is any kind of software to easily do this.
To get more 3D illusions, you can go to Magic Eye.