Always thought these were fun. In Frankfurt we have a museum dedicated only to optical illusions.

Another do-it-yourself one:

Hold a plain, white sheet of paper in one hand and a blue (or red, or green, but the colour needs to be deep. Pastel tones won't work) post-it (roughly the size of the palm of your hand is good) in the other.

Hold them in front of you with outstretched arms and focus on the blue for 30+ seconds. The longer you focus the more intense and lasting the results. Then shift your eyes to the expanse of white and focus.

Done right you should see a white square (the size of the post-it)surrounded by blue paper.