I used to work in a cafe/restaurant in an old mining town from the Gold Rush, and it had a resident ghost.
Overall, he (the owner was adamant the ghost was male) rarely got involved in the day-to-day occurrences of the restaurant, but occasionally he would intervene when things were done that he didn't approve of.
The owner, for instance, had a story of when she first bought the business and was decorating its interior prior to opening. Her business partner had found a framed collection of foreign World War II recruitment posters at a local op shop that he thought would look good on the walls. The ghost, however, wasn't impressed with this choice and when the two returned to the shop the day after hanging all the posters, they found them all on the floor. The Japanese poster's glass frame was broken.
When they re-hanged them that evening, they found the same thing upon returning the next day. This lead the owner to conclude that the ghost may somehow be linked to the War Memorial that's less than a five minute walk from the restaurant. I'm not sure I agree with her, but it's possible.
My own experiences with the ghost weren't so dramatic. One night when I was cleaning the coffee machine at the end of the day, one of the cups (stacked two cups high on top of the machine) fell to the floor. I picked it up and carefully replaced it, only to have it fall off again. I tried again, and held it to ensure it was steady before removing my hand, upon which the cup levitated about 8 inches into the air, hung for a moment, and then fell to the floor. I looked over my shoulder to see the manager staring at me open mouthed, he had seen it too. Wanting to be respectful, and having been told by the owner stories about the ghost before, I apologized aloud to him for repeatedly putting the cup where he didn't want it, and stacked the cup elsewhere on the coffee machine, where it stayed.
The most activity I ever experienced from the ghost was while a certain chef was briefly working at the restaurant. We never figured out why, but for some reason our ghost really didn't like her. During her time in the kitchen, pots and saucepans would move overnight. The CDs she kept at work to play while she was cooking would be mysteriously scratched when she took them from their cases. Sometimes, the metal rack on the wall that the pans sat on would just start shaking quite violently for a few minutes.
Or well-stacked piles of pans would just collapse for no apparent reason. None of which are individually that surprising, but with all of them happening at once fairly often, it was rather odd. I admit, I tried to talk to the ghost about it aloud once or twice (despite other waitresses looking at me like I was an idiot), trying to reassure him that the chef was a good person. But he really just didn't like her. The chef refused to be intimidated, but once she left the restaurant, the strange happenings in the kitchen ceased.
These stories perhaps aren't as scary as some of the other tales on this site, but I always felt as if the ghost only wanted to protect us and keep us safe. And occasionally, just have someone talk to him and give him some attention. I think that, if he could somehow know about it, he would feel content knowing that his presence and "assistance" was known.