KB Piano will turn your computer into a piano and more. It is a free, but ad-supported song editing application that's well-designed, customizable and advanced.
Here is a screenshot to illustrate the features and user interface of this application:
Turn computer into a piano to learn and practice music playing skills. Screenshot of KB Piano - 1067px · 702px
MidiNotate Player
Notation Composer
MuseScore
Synthesia
Virtual MIDI Piano Keyboard