One of the most simple, and effective optical illusions is the Duck-Rabbit.
As a metapicture, the duck-rabbit shifts between two images to the observer, but as the meaning flickers between duck and rabbit in our minds, it is always both, and neither. We are drawn into the picture, becoming a part of it lines, and are asked to choose. Of course, the metapicture knows we will never make up our minds, smiling as it holds on to our attention, until we finally look away.
Recently a new picture has been playing this game, capturing the attention, amusement, and irritation, of eyes glaring at screens all over the globe. Yes, the dress.
While it can be explained by logic, the illusion of the dress became an internet phenomenon because it caused confusion, and provoked questions about images in general, and how we perceive them. It also happened to sprout a fresh bunch of internet memes (interesting work on memes here and here):