MuseScore is a free cross-platform WYSIWYG music notation program
that offers a cost-effective alternative to commercial programs
such as Sibelius and Finale.
You can print beautifully engraved sheet music or save it as PDF
or MIDI file.

Some 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 your keyboard, mouse, or MIDI keyboard
 * Integrated sequencer and FluidSynth software synthesizer
 * Import and export of MusicXML and Standard MIDI Files
 * Available for Windows, Mac and Linux
 * Translated in 43 languages
 * GNU GPL licensed