Mighty Optical Illusions has a great collection of photos which were taken at the right moment for unexpected results. Some of the highlights include perfect moments when the man with the cell phone passes an apparently eavesdropping marble statue, or the sucker punch that Obama appears to be giving John McCain.
Some of them are billboards like the one where vines grew to be the "hair" of a model in the big photo. Some play on light and shadow where one cat appears to have the shadow of a howling wolf. Others appear staged but nonetheless fun to see.
The video is the "Way Cool Scientist" feature from the TV series, "Bill Nye The Science Guy: The Brain episode" where the guest scientist demonstrates how our mind tricks us to seeing things.