MuseScore is open source and free music notation software.

Features:

* WYSIWYG design, notes are entered on a "virtual notepaper"
* TrueType font(s) for printing & display allows for high quality scaling
  to all sizes
* Easy & fast note entry
* Many editing functions
* MusicXML import/export
* MIDI (SMF) import/export
* MuseData import
* MIDI input for note entry
* Integrated sequencer and software synthesizer to play the score
* Print or create pdf files