Here's a nifty illusion to test dominance in your brain hemispheres. According to the lore, if you see the dancer turning clockwise, then you use more of the right side of the brain; if you see the dancer turning counter-clockwise, you are a lefty.
Personally, I see the dancer spinning clockwise, which means I am right-brained. However, if I cover my left-eye and stare at the image with my right eye for about twenty seconds, and then un-focus or narrow my right eye to a slit so the image blurs, the dancer suddenly changes direction to counter-clockwise, meaning my left hemisphere has taken dominance due to greater activity in my right eye (remember, brain hemispheres are contralateral). If I then close both eyes for five seconds and clear my visual memory buffer, when I open them again the dancer is going clockwise again.
Note: It was extremely frustrating for me to figure out how to see the dancer spin in the opposite direction, I couldn't even conceive of how it could be possible. It took me a few minutes to figure out how to establish dominance with my non-dominant side, but once I got it is was easy to reproduce. I would love to hear informal reports on how others fare in this test.