Optical illusions
An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, colour, size, position, tilt, movement), and cognitive illusions, the result of unconscious inferences.
The following are examples of some interesting optical illusions.(to enlarge,click on the pictures)
fahadman 
Wow! So interesting!
You should also note that any and all art/photos can and will be classified as optical illusions that have the ability to give you two perception. All of the above photos fulfill this one, perhaps, univerisal criteria.