The Marauder's Map

"Messrs. Moony, Wormtail, Padfoot, and Prongs
Purveyors of Aids to Magical Mischief-Makers
are proud to present
THE MARAUDER'S MAP"
- The Marauder's Map

The Marauder's Map is a magical document that reveals all of Hogwarts School of Witchcraft and Wizardry. Not only does it show every classroom, every hallway, every corner of the castle, but it also shows every inch of the grounds, as well as all the secret passages that are hidden within its walls.

The Marauder's Map was created by Remus Lupin (Moony), Peter Pettigrew (Wormtail), Sirius Black (Padfoot), and James Potter (Prongs) while they were attending Hogwarts. The map was created between 1975 and 1978, because the titles on the map refer to these four men in their Animagus forms, and as for Remus Lupin in Werewolf form, respectively. This artifact proved quite useful to the foursome (commonly termed Marauders) through the years, though at some point Argus Filch confiscated it.

Source: Harry Potter Wiki