He took out his wand, touched the parchment lightly, and said, "I solemnly swear that I am up to no good." 

And at once, thin ink lines began to spread like a spider's web from the point that George's wand had touched. They joined each other, they crisscrossed, they fanned into every corner of the parchment; then words began to blossom across the top, great, curly green words, that proclaimed: Messrs. Moony, Wormtail, Padfoot, and Prongs

 Purveyors of Aids to Magical Mischief-Makers are proud to present THE MARAUDER'S MAP

 It was a map showing every detail of the Hogwarts castle and grounds. But the truly remarkable thing were the tiny ink dots moving around it, each labeled with a name in minuscule writing. Astounded, Harry bent over it. A labeled dot in the top left corner showed that Professor Dumbledore was pacing his study; the caretaker's cat, Mrs. Norris, was prowling the second floor; and Peeves the Poltergeist was currently bouncing around the trophy room. And as Harry's eyes traveled up and down the familiar corridors, he noticed something else. 

This map showed a set of passages he had never entered. And many of them seemed to lead -

 "Right into Hogsmeade," said Fred, tracing one of them with his finger. "There are seven in all. Now, Filch knows about these four" -- he pointed them out -- "but we're sure we're the only ones who know about these. Don't bother with the one behind the mirror on the fourth floor. We used it until last winter, but it's caved in -- completely blocked. And we don't reckon anyone's ever used this one, because the Whomping Willow's planted right over the entrance. But this one here, this one leads right into the cellar of Honeydukes. We've used it loads of times. And as you might've noticed, the entrance is right outside this room, through that one-eyed old crone's hump." 

"Moony, Wormtaill Padfoot, and Prongs," sighed George, patting the heading of the map. "We owe them so much.