Virtual MIDI Piano Keyboard is a MIDI event generator and receiver. It doesn't produce any sound by itself, but can be used to drive a MIDI synthesizer (either hardware or software, internal or external). You can use the computer's keyboard or mouse to play MIDI notes. You can use the Virtual MIDI Piano Keyboard to display the played MIDI notes from another instrument or MIDI file player.
MW is a low footprint embedded UI framework core intended for use on very limited systems. It embeds the JPEG, Freetype, and Cairo libraries, so it only depends on X. It should be fairly portable, as the only thing you need is to create a window on every platform and manage the events.