Do I believe? Definitely.
Are they disembodied dead souls? I think not.
Have I seen one? Yes, I think.
I believe that "ghosts" are some sort of psychological impression left on a certain spot by some kind of traumatic event or at times by a strong personality. It's something which is always there but can only be perceived by certain folks - so everyone who enters a haunted house won't see the ghost. And I don't think poltergeists are true "ghosts" at all. I'm a firm believer in psychic phenomena, as if you can't tell.
My story? One night (probably 4-5 years back now) I awoke from a very horrid, vivid nightmare at about 3am. I can't recall what it was about - just that it was terrifying. Anyway, when I awoke I was paralyzed, and could see the shape of a tall black man in the corner of my room. Not black like an African, mind you - but truly black, like a shadow that was merely darker than the other shadows. It's hard to describe, but I could clearly see his outline. It was a tall thin man, with his arms pressed firmly to his sides. I for some reason got the impression of his wearing a suit. I've also at other times heard odd noises in my house, such as footsteps passing behind me through the kitchen into the bathroom.