Piano Training for 3DS is a simple homebrew app for the Nintendo 3DS to practice piano on the go.
Press the corresponding key on the touch screen of the note displayed on the top screen. If the wrong key is pressed it will light red. Use the bottom buttons to change to treble or bass modes.
http://3dbrew.org/wiki/Setting_up_Development_Environment. You will also need Python 2.7 in order to generate icon and banner files for .cia and .3ds outputs (3DS Banner Maker). To run on actual hardware you will need one of the available methods to run homebrew like Ninjhax or Gateway 3DS.
Have fun!