Fat Friar: The Ghost of Hufflepuff

Haila Ravenclaw (also known as Lady Gray): Ravenclaw's ghost.

The bloody baron: the ghost of Slytherin

Binns Cuthbert: The history of magic teachers.

Myrtle crying: the victim of the basilisk

Sir Patrick: the leader of sleepy hollow hunting

Peeves (not exactly a ghost, because Peeves can touch the real thing)