MuseScore is a free, cross-platform, WYSIWYG music-notation program, licensed under GNU GPL.
Highlights:
- 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
- Audio playback of the score
- Import and export of MusicXML and standard MIDI files
- Available in more than 50 languages