It’s all a matter of perspective.
Pespective illusion
Square angles not so square
Would you say that the branches are crossing at square angles?
I did not think so. But they actually do despite the illusion:
From LottoLab Studio.
Pisa tower
A very usual picture for tourists in Pisa (Italy), with a variety of options.
Heavy lifting
Blue blocks
Despite what you may think the blue blocks appearing at the top of the cube are the same colour on both images.
Eclipse of Mars
Start by looking at the white dot. For a long time. Then, move backwards slowly and you will notice that the red circle will disappear progressively.
One grey for all
On the drawing below, the grey colour of the squares marked as A and B is exactly the same. Still…
Perception of lightness
It is not often that you find the most simple and lighter illusions described as actively than on the web site for the MIT (Massachussets Institute of Technology).
Lightness Perception and Lightness Illusions
interactive movies based on a paper by Edward H. Adelson
Anamorphosis
Anamorphosis is a technique to hide images in another image. It has long been used by painters. And some of them are quite well known like the The Ambassadors (Hans Holbein the Younger, 1533) below.
In this painting, there is a highly deformed skull appearing like a whitish blade at the bottom of the painting.
Learn everything about anamorphosis from Jessica Palmer in ScienceBlogs.