NtEd - Music Score Notation editor.
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.
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.
Orpheus App - Sheet Music Reader for Android
music21 - Music21 is a Python-based toolkit for computer-aided musicology.