Synthesia - A fun way to learn how to play the piano.
MilkyTracker - MilkyTracker is an open source, multi-platform music application for creating .MOD and .
Playground Sessions - Online piano learning game.
OpenMPT - OpenMPT is a popular tracker software for Windows.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
FamiStudio - FamiStudio is very simple music editor for the Nintendo Entertainment System or Famicom. It is designed to be easier to use than FamiTracker, but its feature set is also much more limited.