free graphical music notation editor (GTK front end to LilyPond)

 Denemo is a free music notation program GUI for GNU/Linux, macOS and Windows
 that lets you rapidly enter notation which it typesets using the LilyPond
 music engraver. Music can be typed in at the PC keyboard, or played in
 via MIDI controller, or input acoustically into a microphone plugged into
 your computer’s soundcard.
 .
 Denemo uses LilyPond which generates beautiful sheet music to the highest
 publishing standards. During input Denemo displays the staffs in a simple
 fashion, so you can enter and edit the music efficiently. The typesetting
 is done in the background while you work, and is generally flawless
 publication quality. Some final tweaks can be done on the typeset score
 with the mouse if needed. This represents an enormous practical improvement
 over the popular programs which require you to re-position colliding
 notation constantly as you enter the music.
 .
 Beginner to Professional: Denemo can be used for a brief student homework
 all the way to a full-scale opera. Support is there for creating the
 widest range of notation – Tablature, Chord Charts, Fret Diagrams, Drums,
 transposing instruments, ossia, ottava, cue, title pages, table-of-contents,
 critical commentaries, footnotes quoting music, and more with LilyPond’s
 extensive syntax available for even more demanding uses.