I personally haven't seen one, but I've felt a certain presence in two separate properties. Sometimes ghosts aren't things you can see, but you can sense them in other ways - sometimes it's just a heavily negative feeling in a place, but I'm more sensitive to these things that most people.
The first property is a place in Belsize Park - a house that a friend of mine lived in for a year. I didn't see anything but it was clear something bad had happened there and the vibes still lingered. I always felt uncomfortable going into that place, it had a very bleak, sad feeling as soon as you walked in.
The second is the house next door to where my my mum lives. In the early 80s a baby died in the back room of the house. Now, every time you go in you can feel this nasty tense atmosphere which intensifies as you go into the back room. Four families have lived in the house since then, each one has done serious redecorating and building work to achieve the right feel and, I believe, remove this sensation that they couldn't put their finger on. What adds weight to the story is that fact that Andrew, a kid I knew from the first family that moved in after the tragedy, said he was kept awake some nights by a baby crying downstairs, when the family didn't have a baby.
Nothing amazing there I know. My aunt is a medium though, but I have declined her offers to join her circle because it's not really something I want to get involved in.