Notation Pad - Notation Pad is the Easiest way to write sheet music with notations on iOS/Android.
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.
Orpheus App - Sheet Music Reader for Android
Harmony Assistant - Harmony Assistant is an unbeatable software for computer-assisted music composition and editing.
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.