Room of Requirement
"Sometimes it is there, and sometimes it is not..."
The Room of Requirement is a magical room which can only be discovered by someone who is in need. It is called the Come and Go Room by the house-elves of Hogwarts. The Room is located on the seventh floor, opposite a tapestry showing Barnabas the Barmy trying to teach trolls to dance the ballet. To make the Room appear, a person has to walk past the section of blank wall three times concentrating hard on what is needed.
Dumbledore once happened upon the Room of Requirement early one morning when his bladder was especially full. When he found it, it was full of chamber pots. (GF23)
Harry found a room ready for teaching Defence Against the Dark Arts lessons. It contains bookcases, cushions, and even, when Harry suddenly realized he needed one, a whistle (OP18). Harry discovered, just before Christmas, that Dobby had decorated the room with Christmas ornaments bearing Harry's face and the words "Have a Very Harry Christmas!" The house-elf had also hung mistletoe in the middle of the room, and after the D.A. meeting, Harry had his first kiss under it (OP21).
Dobby reports to Harry that Draco has been using the room. (HBP21) Harry realizes that it has never appeared on the Marauder's Map, and Hermione surmises it is Unplottable. It is impossible for Dobby to follow Draco inside, or for Harry to get in without knowing what purpose the room is serving at the time.
Desperate for a place to hide the Half-Blood Prince's copy of Advanced Potion-Making, the Room became the size of a large cathedral, with high windows. It looked like "a city with towering walls", full of objects hidden by generations of Hogwarts inhabitants. (HBP24)Items Harry noticed include:
Harry hid the Book in a large cupboard that's surface seemed to be blistered by acid; behind a cage which held a skeleton of a creature with five legs.
Sibyll Trelawney uses the Room of Requirement to hide her empty sherry bottles. On one of her visits she encountered the Room occupied by Draco Malfoy who had just successfully fixed the Vanishing cabinet. He used Peruvian Instant Darkness Powder and threw her out of the Room, where Harry was in the corridor. (HBP25)