Virtual MIDI Piano Keyboard - Virtual MIDI Piano Keyboard is a MIDI events generator and receiver.
Synthesia - A fun way to learn how to play the piano.
Free Piano - FreePiano is a virtual MIDI keyboard and a VST host for windows.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
MidiKeys - MidiKeys is a small application for Mac OS X that presents a miniature MIDI keyboard onscreen.
Playground Sessions - Online piano learning game.