The list of famous people who stayed at the Parker House is astounding. Charles Dickens gave his first public reading of A Christmas Carol while a guest there. While staying at the Parker House, actor John Wilkes booth practiced his pistol-shooting at a nearby firing range. Years later, a widowed Mary Todd Lincoln was a guest as well.
From politicians (Ulysses S. Grant, Ross Perot, JFK) to actors and celebrities (Judy Garland, the Grateful Dead, Emeril Lagasse), scores of well-known people have stayed at the Parker House.
However, the identity of its long-time ghostly guests is less certain.
A man dressed in Victorian clothing has been seen on the ninth and tenth floors. He appears solid for only a moment before fading away. Hotel lore says it is the ghost of found Harry Parker. Since this ghost once appeared at the foot of a guest's bed and asked if she liked her room, that seems like a good guess. Perceptive visitors to the ninth and tenth floors have also reported seeing orbs of white light floating in the hallway.
Apparently the third floor also has its share of supernatural activity. For many years the elevator would invariably stop on this floor, even when no one was visible. The famous actress Charlotte Cushman often stayed on the third floor, and died there in 1876. Hotel lore naturally blames the faulty elevator on her post-mortem dramatics, but I think the jury's out on which ghost is to blame.
On the same floor, guest room 303 was allegedly where a traveling liquor salesman killed himself. Guests and staff alike complained about the strange laughter heard in the room, and scent of whiskey that never left. To add to the general creepiness, strange shadows were seen on the walls and the water in the bathroom would turn itself on and off. Rather than rent out the room to thrill-seekers, the hotel management took a different approach and turned the room into a storage closet. This stopped the supernatural activity. Not even a suicidal liquor salesman wants to spend eternity haunting a closet!
The information about famous guests is from Susan Wilson's The Omni Parker House. A Brief History of America's Longest Continuously Operating Hotel. I found the ghost stories in Cheri Revai's Haunted Massachusetts.