An example of "Troxler fading"

Fix your gaze on the center of this image (the black pixel) and stare at it for about 20 seconds.  The colors will disappear.

Troxler's fading has been attributed to the adaptation of neurons vital for perceiving stimuli in the visual system. It is part of the general principle in sensory systems that unvarying stimuli soon disappear from our awareness. For example, if a small piece of paper is dropped on the inside of one's forearm, it is felt for a short period of time. Soon, however, the sensation fades away.

An even more dramatic example is in the video at Digg.