You know how you can use the Nintendo 3DS as a controller for Super Smash Bros. for Wii U? The catch is that you need a copy of both the Wii U and Nintendo 3DS versions of the game for this to work. Or rather, you used to.
Today, Nintendo released a $0.99 app on the Nintendo eShop titled Smash Controller, and it lets you use the 3DS as a controller for the Wii U version of Smash without owning the 3DS version of the game.
You can download the app off the 3DS eShop.