MuseScore is a free cross platform WYSIWYG music notation
program, licenced under GNU GPL. Features:
  * WYSIWYG, notes are entered on a "virtual note sheet"
  * unlimited number of staves
  * up to four voices/staff
  * easy and fast note entry with mouse, keyboard or midi
  * integrated sequencer and FluidSynth software synthesizer
  * import and export of MusicXml and standard midifiles (SMF)
  * platform independent code for Linux, Windows and Mac

jack-audio-connection-kit and portaudio are optional dependencies.