Paper Mario: Sticker Star Cheats

Luigi Locations
In the background of certain levels, you can spot Mario's brother Luigi, who can be peeled off the level by Paperizing. If you peel him off in all 5 locations, he'll appear during the credits sequence. In addition, each individual peeling will unlock a rather amusing newspaper article in the leftmost house in Decalburg's Residential Area.

Luigi Location: World 1-6: On the bridge leading to the central tower, look in the backgrond.
Luigi Location: World 2-5: On the fourth floor of the tower, in a window on the far right end of the second outdoor walkway.
Luigi Location: World 3-12: On a rock formation sticking out of the ocean in the background that's only visible from the pier.
Luigi Location: World 4-5: Near the end of the chairlift after the enemies stop coming. You only have so much time, so be quick.
Luigi Location: World 5-5: In the background to the left of the hot springs.

Hidden Song at Game's End
After beating the game (and watching the credits), the screen will shift to the book from the start of the game. The book closes and the light turns off. At this point, just wait about three minutes. A hidden lullaby song will start to play, and will continue to play until you hit a button.