Free Piano is a free and open source piano playing application that's portable and well-designed.
Here are some screenshots to illustrate the features and user interface of this application:
Is a piano that play using keyboard and change the interface to display a music sheet. Screenshot of Free Piano - 752px · 400pxFree Piano: User interface. Screenshot of Free Piano - 752px · 400px
MidiNotate Player
Notation Composer
MuseScore
Synthesia
Virtual MIDI Piano Keyboard