Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Synfire - The comprehensive Music Prototyping Studio for professionals and enthusiasts.
MuseScore.org - Create, play back and print beautiful sheet music with free and easy to use music notation software MuseScore. For Windows, Mac and Linux.
MMA - “MMA—Musical MIDI Accompaniment” is an accompaniment generator.
Finale - Finale, the world standard for music notation software, lets you compose, arrange, notate, and print engraver-quality sheet music.
Sundog Scale Studio - The MIDI chords and melodies laboratory