I thought this was most interesting: from 9 mysteries that science cannot explain:
From the Shakespeare play "MacBeth" to the NBC show "Medium," spirits of the dead have long made an appearance in our culture and folklore. Many people have reported seeing apparitions of both shadowy strangers and departed loved ones. Though definitive proof for the existence of ghosts remains elusive, sincere eyewitnesses continue to report seeing, photographing, and even communicating with ghosts. Ghost investigators hope to one day prove that the dead can contact the living, providing a final answer to the mystery.