NtEd - Music Score Notation editor.
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.
MMA - “MMA—Musical MIDI Accompaniment” is an accompaniment generator.
Guitar Pro 7 - Create, play and share your tabs
music21 - Music21 is a Python-based toolkit for computer-aided musicology.