A Map for Marauders

I've been on a Harry Potter binge lately, in anticipation of Halloween and the end of Pottermore's beta phase - if it ever does. Silly me for not willing to attempt the Magical Quill challenge! With that lack of ambition, I doubt I'll be sorted into Slytherin! Boo hoo, personally I'm hoping for Ravenclaw or Hufflepuff, the latter basically because I like yellow.
One of my favorite books in the series is Prisoner of Azkaban for two reasons: first, it introduces the character of Sirius Black, who is one of my favorite characters in the story. The second reason is The Marauders Map!

Harry is introduced to this intriguing sheet of parchment by way of his friend Ron's mischievous brothers, Fred and George. They themselves happened to pull it out of a drawer in Filch's office. I don't want to spoil too much about the cartographers' backgrounds for those who have been completely unaware of the Potterverse this last decade, just that Moony, Wormtail, Padfoot and Prongs are all quite important characters. Also Wormtail's a jerk.
The map appears to be a plain sheet of parchment, but if you solemnly swear that you are up to no good, the entire labyrinth of Hogwarts and surrounding grounds appears, with labeled dots representing people. Harry uses the map throughout the rest of the series to sneak into Hogsmeade, sneak away from Snape, stalk Ginny, and more! The map also has the ability to spew snark, as shown when Snape tries to make the map appear on the parchment without using the "magic words." I'd love to have a GPS with that feature!