At first this one may be hard to see, but if you begin to scan back and forth across the image you will notice that the squares in your periphery begin to rotate. As soon as your eyes stop moving, however, rotation will cease.
No, this is not an animated GIF. The image really is static. Notice that when you look at any individual point dead on, it will stop moving. This powerful illusion is derived from interacting color contrasts and shape positions within the image.
This is a classic illusion named after Ludimar Hermann who discovered it in 1870. At every point where the white lines intersect our eyes perceive a gray, shadowy blob. If you look directly at one of the intersections though, the blob disappears.