WARNING: More references to Harry Potter !
The Room of Requirement is a special place at Hogwarts which only appears if you really need it. For Dumbledore, it was a chamber full of chamber pots. I wish it would be possible to run into one of those when you are kilometers away from home and Bucharest has so little public toilets!
Marauder's Map is a map that shows you where everyone is at any time. Working in a rather big school and everyone being busy all the time, it's difficult to find the people you need most of the time. So this would come very much in hand. And when they found me, they would say :"ACCIO Daiana!"
Click on the picture above for one of the most complete websites about Harry Potter. It is based mostly on the books, which I appreciate and also recommended by J.K. Rowling herself.