Wikipedia
Denemo
Denemo is a free software graphical interface for music notation, mainly to GNU LilyPond, a program for engraving musical scores. Denemo has been under development since 1999. Using GTK+ 2 or 3, it works on Linux, Microsoft Windows and OS X.
Denemo helps prepare notation for publishing and lets a user rapidly enter notation, simultaneously typesetting via the LilyPond music engraver. Music can be typed in using a PC keyboard, taken from MIDI input, or played into a microphone plugged into a soundcard. The program plays back via an internal sampler and can act as a JACK/MIDI client. Denemo includes scripts to run music tests and practice exercises for educational purposes.