Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
JJazzLab - Fed up with boring backing tracks ? Try JJazzLab!
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.
Band-in-a-Box - Band-in-a-Box is so easy to use!
Finale - Finale, the world standard for music notation software, lets you compose, arrange, notate, and print engraver-quality sheet music.
NtEd - Music Score Notation editor.