NtEd - Music Score Notation editor.
Chordsheet Maker - Create chord sheets or lead sheets quick and easily from your iPad. Add rhythms, endings, segno, codas and all what's necessary for your musicians to know what's your music about. Transcription made easy!
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.
Tonelib JAM - Create, edit, print and listen to tablature and guitar sheet music
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Aria Maestosa - Aria Maestosa is an open-source (GPL) midi tracker/editor.