MuseScore

MuseScore is an excellent WYSIWYG music notation program for multiple operating systems, licensed under the GNU GPL.

Main Features:

• WYSIWYG music editor, notes are inserted into a "virtual music sheet"
• Unlimited number of sheet music
• Up to four voices per outline
• Simple and fast note entry using mouse, keyboard or MIDI
• Includes FluidSynth sequencer and synthesizer
• Import and export MusicXML and standard MIDI files
• Available for Windows, Mac and Linux
• GNU GPL license.