Have seen things twice, and heard things once...
Most recently in 2004, staying in a 17th century house in Bermondsey, London for a few months while studying, I came in one night, opened the front door and out between me and the door dashed a very distinct, and distinctive, black cat with orange stripes! It had very pricked up, pointy ears and its fur was all on end

I quickly looked outside to see where it went but it was nowhere...needless to say the house didn't own any cats.
About 15 years ago, first trip to England for a working holiday, staying in 18th century terrace in Earl's Court (of course!), we would frequently wake in the night to the sound of quick footsteps on the landing outside the bedrooms doors. My friend and I slept in the lounge room, and the steps always came up to the doorway of the lounge, but no-one ever came in, and no-one ever went back the other way. There were 10 people in the flat all up, and they all heard the footsteps.
I've seen the figure of a man in the conservation lab at the National Archives here in Canberra, although what he was doing there I've no idea, as the building is only 30 years old. We theorised that he was something to do with our first Parliament, because a large composite photograph of the members of that Parliament was in the lab at the time, being checked and treated.