Star Coin

A Star Coin is a collectible found in New Super Mario Bros. Three star coins appear in each level (be it numbered, lettered, towers, ghost houses or castles). Some are easy to grab, while others require special power-ups (such as the Mini Mushroom or the Blue Koopa Shell) to access. There are eighty levels, thus there are 240 Star Coins to find in the game. Each Star Coin adds 1000 points (the same amount of points given as collecting an extra Fire Flower, Mushroom, Mini Mushroom, Blue Shell, or Star) to the player's game score.

They also appear in New Super Mario Bros. Wii.

Uses
New Super Mario Bros.
 * Opening signs on the world map. Little picket signs block bonus paths that may lead to Toad Houses or to alternate, lettered levels. (e.g. 1-A) There are thirty-two such signs in the game, each requiring five Star Coins to open. Also, after opening the signs, the player is given the option to save, allowing them to easily save their progress instead of having to play through a tower or castle.
 * Buying new backgrounds. After beating the game once, a blue Toad House will appear north of Level 1-1. There are four bonus backgrounds for the touch screen that cost twenty Star Coins each (the last background can be unlocked by opening every Toad House).
 * Note: Once all 240 star coins have been collected, a star will appear next to the save file it was completed in and on the map screen next to the symbol, the message will display "All Spent".

New Super Mario Bros. Wii
 * Buying "hint movies" at Peach's Castle.
 * When all Star Coins in a world are found, a level in World 9 (unlocked after beating final boss) is unlocked.