NtEd - Music Score Notation editor.
Flat - Online collaborative music score and tab editor, accessible from any device
Sibelius - Sibelius is a virtual score creation tool which allows composers to easily create new piano scores, developed by Avid.
Musicnotes - Access your music sheets anywhere
music21 - Music21 is a Python-based toolkit for computer-aided musicology.
Musopen - Unlimited royalty free music sheets, tracks and textbooks 🎶