Sir Nicholas de Mimsy Porpington, Helena Ravenclaw, the Bloody Baron and the Fat Friar. Cuthbert Binns, Moaning Myrtle, Sir Patrick Delaney-Podmore, Edgar Clogg, Peeves.
There's a group called The Headless Hunt as well. They gallop around on ghost horses with their heads tucked under their arms.
The ghost, Cuthbert Binns is the professor of History of Magic. Would any teacher want to stay on the job that long?