Lilypond

From P2P Foundation

URL = http://lilypond.org/

GNU LilyPond is a computer program for music engraving. One of LilyPond's major goals is to produce scores that are engraved with traditional layout rules, reflecting the era when scores were engraved by hand.

LilyPond is cross-platform, and is available for several common operating systems; released under the terms of the GNU General Public License version 2, LilyPond is free software.

--Wikipedia