Synthesia

Synthesia is a video game and piano keyboard trainer for Microsoft Windows and Mac OS X, which allows users to play a MIDI keyboard or use a computer keyboard in time to a MIDI file by following on-screen directions, much in the style of Keyboard Mania or Guitar Hero. It was originally named Piano Hero, but due to the similarity of gameplay with Guitar Hero; however, Activision (the owners of the rights to Guitar Hero) sent a cease and desist to the program's creator, Nicholas Piegdon.