Starngely, lots of people at the Telecom where I work swear to have seen and even pushed by spirits such as these.
But few of them are very scientific people.
I personally have not had any ghostly encounters, and it would kinda go against my religious beliefs if I did.
A friend of mine, who is a staunch atheiest / Humnanist, did have an interesting encounter with an apperition the day after his Father died (he was in second grade at the time).
His account seems quite credible, especisally since he is a big James Randi admirer.
(he also doesn't talk about it much probably becuase of that)