Faded Dots Illusion
What to see and do: You should see sixteen squares with a fading dot in each intersection of four squares. If you look directly at the dot, it disappears. Try moving the squares around. If they are too far apart or too close together, the illusion will not work.
What's happening: There are white lines between the black squares. Your brain expects the lines to be extra bright where they cross each other. Since they are not "extra bright" you brain sees them as darker than they should be.
Credit: This illusion is based on an illusion from a book called "Incredible Visual Illusions" by Al Seckel.