essay: Hogwarts Ghosts by Mike Gray

There are at least twenty ghosts at Hogwarts (SS7).
When ghost-related decisions must be made, they hold a "ghost's council" (GF12)
The ghosts are pearly white (except Peeves) and glow (PA9)
Ghosts communicate with each other by ghostly letters. Nearly Headless Nick received one from Sir Patrick of the Headless Hunt. He must have used something of that sort to invite people to his deathday party, too. (CS8)

known ghosts of Hogwarts:

Nearly Headless Nick (Gryffindor ghost) (SS7, CS

The Bloody Baron (Slytherin ghost)
The Bloody Baron is a grim, silent, terrifying fellow who is covered with bloodstains. He is never heard to speak, although Harry does impersonate him once, using a hoarse whisper (SS16). No one is quite sure about the source of the bloodstains [1], and no one dares ask. Peeves, who respects the Baron for some unknown reason, calls him "Your Bloodiness" and "Mr. Baron." 
In the film, the Bloody Baron is inexplicably depicted as rather frilly and goofy, not at all like the character in the books. (SS/f)

The Fat Friar (Hufflepuff ghost)
Was once a Hufflepuff, now is their resident ghost. He is a jolly, friendly ghost who cheerfully wishes the first years good day and hopes they'll be in Hufflepuff (SS7).
Moaning Myrtle
Myrtle haunts a girls' bathroom on the second floor. She does leave the bathroom once in a while. For example, she sometimes turns up in the Prefects' Bathroom. She occasionally gets flushed out of the castle with the contents of a toilet and has floated around in the Lake.

Peeves the poltergeist
Many of the ghosts do no consider Peeves to be a ghost. He causes no end of trouble and delights in mayhem and chaos.

The Grey Lady (Ravenclaw ghost) (SS12)
Very little is known about the Grey Lady apart from the fact that she is very tall and she is the resident ghost of Ravenclaw. Harry and Ron encounter her gliding past in the corridor while out looking for the Mirror of Erised (SS12). 
So how do we know that the Grey Lady is the Ravenclaw ghost? JKR showed a page from her notebook in a televised interview. Clearly visible on that page was the list of house ghosts which indicated that the Ravenclaw ghost was the Grey Lady. In the film, she appeared a number of times. She was sitting at a desk in MacGonagall's classroom when Harry, Ron, and Hermione came bursting in, demanding to talk to Dumbledore. She was also talking with Nearly Headless Nick when he told her that Harry had been made a Seeker  (SS/f)

A reader sent this note:
"I thought you'd be interested to know that the ghost of Ravenclaw, the Grey Lady, could actually be or be based on the 'Grey Lady' that haunts Brodick Castle, on the Isle of Arran in Scotland. The castle was originally built by the Stewarts and was captured by the English during the Wars of Independence. In 1406 an English fleet sailed into the Clyde and destroyed much of the original castle. For much of its life the castle has been owned by members of the Hamilton family. The older part of the castle is said to be haunted by the "Grey Lady", who starved to death in the dungeons of the castle because she had the plague."

Professor Binns
Binns is the only Hogwarts teacher who is a ghost.

non-Hogwarts ghosts
 

[1] -Naturally, we all assume that the "silvery" bloodstains are that color because the Baron is a ghost. But it bears mention that Unicorns possibly have silvery blood as well, judging by descriptions of bloodstains in the forest when the unicorns were being killed. So there is a chance that the Baron is covered in Unicorn blood.