NtEd - Music Score Notation editor.
DaCapo Editor - DaCapo Editor is a free Open Source music sheet editor which allows to easily create nice looking partitions. It is available as a web application as well as a desktop application for Windows, MacOS and Linux.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
ABCexplorer - ABCexplorer is a musical creation utility that comes with advanced support to create, edit, play, convert and manage musical files with ABC text format support.
LilyPond - GNU LilyPond is a computer program for music engraving.