Star Pieces are items found in Paper Mario (160 in all) and Paper Mario: The Thousand-Year Door (100 in all). They are found in levels, backgrounds, trees and given to Mario by various characters in the game. There are badges in both games that alerts Mario when a Star Piece is near. In both games, there are platforms that flips to reveal a Star Piece when you ground-pound on it. Chuck Quizmo in Paper Mario is a primary source of Star Pieces, giving a total of 64 in the entire game. In Paper Mario at Mario's house, there is a scoreboard that tells you how many star pieces you've collected and how many more you need to find.
Star Pieces are used in both games to trade for badges. Merlow in Paper Mario trades this way in Shooting Star Summit and Dazzle trades this way in Rougeport Sewers. You need to trade them Star Pieces in order to get exclusive badges if you want to dominate the game. Ms. Mowz in Paper Mario: The Thousand-Year Door tells you if a treasure is near-by if you use her ability, and treasures include Star Pieces. Merluvlee can tell Mario where some Star Pieces are if he so choses.
- In Mario Party 5, there is a mini-game that involves collecting star pieces. Mario Party 5 also shows the Star Spirits quite often, so it is entirely possible that they used Paper Mario star pieces for their mini-game.