It goes without saying that George is probably the most famous resident ghost of Disney world. If I might add to the rumors though, I have also heard that George died during the construction of the Haunted Mansion, same stories, different attraction. I've also read that (whichever ride it may be POTC/HM) cast members have to say 'Good morning George' as the ride opens or the ride will malfunction all day, and same goes at closing 'Goodnight George' or the ride will malfunction all the next day.
Another one has also already been mentioned, the girl that rides Spaceship Earth.
At night time or after closing as everyone is leaving the park (Magic Kingdom) there is said to have been some sightings of a tan figure walking in front of the castle. Some say it's Walt, but if Walt was still roaming around i'd think it would be in
Disneyland. Who knows though?
There is also a story among cast members that around the Tower of Terror at off hours there is a man walking around, usually the wrong way (about to enter a cast member only area near the gift shop area) he does not respond to calls and disappears.
I've even read of a couple that was thought to be seen swimming towards the old Discovery Island and when security catches up to where they should be they are not there.
And of course there are countless sightings and stories of orbs and apparitions walking around Main Street USA, Tomorrowland, Frontierland, my favorites though come from the Haunted Mansion.
It is supposedly not un-ordinary for family members or friends to dump the ashes of the recently deceased into the Haunted Mansion. I've found a website before where cast members who have worked at the Haunted Mansion (and I will try to find it and post it here) have talked about how creepy the Haunted Mansion actually is.
A few instances..... Before the new load of people come out of the stretching room and the cast member(s) are by themselves they sometimes feel their hair being tugged on, or the feeling of somebody bumping into them, and some claim to have even seen an old man in one of the doom buggies coming back through and when they call the exit cast member to ask about him, they say they never sent anybody through. Also, there is supposed to be a little girl that sometimes exits the ride at the end and disappears before getting to the end of the moving sidewalk.
I hope these stories and accounts have added to the conversation, and as soon as I find the website(s) i'll post them ASAP.