The Bloody Baron: (See Slytherin)
The Fat Frair: (See Hufflepuff)
The Grey Lady: (See Ravenclaw)
Moaning Mrytle: Most of the time you can find her in the girls bathroom. She sometimes leaves her stall to visit the perfects bathroom :)
Peeves the Poltergeist: Some people don't even consider him a ghost because he is in color. He causes so much mayhem and chaos.
Professor Binns: He is the only Hogwarts ghost, for a teacher.