Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Synthesia - A fun way to learn how to play the piano.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
Playground Sessions - Online piano learning game.
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.