SoundGrail - SoundGrail Music App designed for pianists, guitarists, and DJs.
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
keezy - A colorful soundboard. Play with music.
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
Splice - Music creation, collaboration, and sharing made simple.
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.