MuseScore is a free cross platform WYSIWYG music notation program, licenced under GNU GPL.
Some highlights: Screenshot
* WYSIWYG, notes are entered on a "virtual note sheet"
* Unlimited number of staves
* Up to four voices per staff
* Easy and fast note entry with mouse, keyboard or MIDI
* Integrated sequencer and FluidSynth software synthesizer
* Import and export of MusicXML and Standard MIDI Files
* Available for Windows, Mac and Linux
* Translated in 26 languages
* GNU GPL licenced
Link Download:
Official Website
