NtEd - Music Score Notation editor.
EarMaster - Professional software for ear training and practical music theory on Windows and Mac.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Meludia - Listening exercises and to sharpen your ear and music theory skills
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.
GNU Solfege - GNU Solfege is free music education software.