Flat - Online collaborative music score and tab editor, accessible from any device
NtEd - Music Score Notation editor.
Musicnotes - Access your music sheets anywhere
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.
Musopen - Unlimited royalty free music sheets, tracks and textbooks 🎶
music21 - Music21 is a Python-based toolkit for computer-aided musicology.