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. Additionally, Synthesia can be paired with MIDI keyboards that have illuminated keys, or with virtual player piano on screen, which some people believe makes learning piano easier for beginners. It was originally named Piano Hero, due to the similarity of gameplay with Guitar Hero; until Activision (the owners of the rights to Guitar Hero) sent a cease and desist to the program's creator, Nicholas Piegdon.
History
Synthesia was started around 2006 by Nicholas Piegdon, originally named "Piano Hero". Hosted as an open source project on sourceforge, it was released under MIT license.
The program was originally Windows-only, but after a donation-drive in early 2007, the game was ported to Mac OS X.
Cease and desist letter