NoteAbilityPro is a professional music notation package for the Macintosh OS X operating system. Developed by Dr. Keith Hamel at the University of British Columbia, it is easily the most sophisticated music notation software available on any platform. NoteAbility combines both musical intelligence and graphical flexibility in a direct and intuitive graphical user interface. Notate anything from simple melodies to complex avant garde orchestral music, play the score on your MIDI synthesizer or using Quicktime Musical Instruments and print a publishable copy of your score on any OS X compatible printer.
http://debussy.music.ubc.ca/NoteAbility/