Visual psychologists love illusions, because they provide a means of understanding how the brain and the visual system interact. Here are a few well known ones.
The size of the surrounding circles affects our perception of the size of the centre circle
Another disruption of our perception of parallel lines: this time by horizontal and vertical interference
We tend to fill in the contours of the illusory triangle, rather than completing the circles
Muller-Lyer illusion: most people see the vertical line on the left as longer than the one on the right
More illusions can be found at http:\\michaelbach.de