MidiKeys shows a little keyboard on screen, and you can use the computer keyboard to play MIDI notes. 'z' through ',' is the octave starting a middle C (note 60). The keys on the row above are flats (black keys), and the octave below is on the row starting with 'q' (and the digit row is that octave's flats).
In addition, you can select a MIDI source and see incoming notes played on the keyboard.
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