Marauder's Map

James Potter, Sirius Black, Remus Lupin and Peter Pettigrew (known as the Marauders) created the Marauders Map while they were students at Hogwarts. The map is a magical document which exposes every part of Hogwarts, including the secret passages, as well as the location of every person on the grounds, indicated by a dot and their name (though in the movies footsteps were used as indicators instead). The map can’t be fooled by invisibility cloaks or any kind of potion or spell, and can even detect the castle ghosts. The only flaws of the map are that it’s unable to discern the identities of people with similar names (like Barty Crouch Jr./Sr.), and it doesn’t show rooms that are unable to be plotted on a map (like the Room of Requirement).