Star Coin

Star Coins are collectibles found in New Super Mario Bros. and New Super Mario Bros. Wii. Three Star Coins appear in each level where some are easy to collect, while others require special power-ups such as the Mini Mushroom or the Blue Shell to access.

In New Super Mario Bros. there are eighty levels, thus there are 240 Star Coins to find in the game. Each Star Coin adds 1,000 points to the player's game score.

They also appear in New Super Mario Bros. Wii. In this game, there are 231 Star Coins. Three Star Coins appear in each of the game's seventy-seven levels much like New Super Mario Bros. Also, collecting one adds 4,000 points to the player's total score, instead of 1,000 like in its predecessor.

Most recently, they have appeared in Super Mario 3D Land, though with a slight name change to Star Medals.

Although they did not appear in Super Mario Galaxy 2 directly, very similar items called Comet Medals appear.

And very similar to them were Star Medals, that appeared in SM3DL.

Uses

 * One use is for opening Star Coin Signs leading to Toad Houses or bonus letter levels (e.g. 1-A or 5-B). There are 32 of these signs in the game, each requiring 5 Star Coins to open. After Mario opens the signs, he is has an option to save, allowing him to easily save his progress without completing an uncompleted tower or castle before completing the game.
 * Another use is for buying new backgrounds. After completing the game at least once, a blue Toad House will appear north of level 1-1. There are 5 bonus backgrounds for the Touch Screen that cost 20 Star Coins each. The 5th background can be unlocked by opening every Toad House.
 * Once all 240 Star Coins have been collected, a 2nd star will appear next to the save file. There will also be a message on the Touch Screen next to the Star Coin symbol reading "Complete".
 * Spending all the Star Coins not only unlocks the final Touch Screen background, but also leaves a message next to the Star Coin symbol on the top screen in the world map reading "All Spent".

Uses

 * One use is for buying Hint Movies at Peach's Castle.
 * Another use is collecting all Star Coins in a world so a level in World 9 (unlocked after beating final boss) is unlocked. It will be the level number corresponding to the world number (e.g. collecting the Star Coins in World 1 will unlock 9-1).
 * The final use for a Star Coin is to cause a star to be displayed on the File Select Screen. If the player collects all twenty-four Star Coins in World 9, he or she will earn a star. Collecting all Star Coins in the game will also result in getting a second star.

Super Mario 3D Land
In the Nintendo 3DS game, Super Mario 3D Land, the equivalent of Star Coins are Star Medals. There are three in every level, like in the New Super Mario Bros. games. They are used to unlock other courses. The Star Medals have a similar appearance to Comet Medals from Super Mario Galaxy 2, but they have stars on them instead of comets. There are 285 Star Medals in the game's levels (including the Special Worlds), plus an unknown number of Mystery Box medals, though it is known that there are at least 50. It is confirmed that through Streetpass mystery boxes the player can obtain 999 Star Medals although this serves no purpose as only 290 are needed to unlock all levels. When the player collects a Star Medal they have already collected, it becomes three coins. Also, when the player opens a course with Star Medals, the players pay their medals, but they are refunded back to the player afterwards.