Flat - Online collaborative music score and tab editor, accessible from any device
NtEd - Music Score Notation editor.
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 🎶
ABCexplorer - ABCexplorer is a musical creation utility that comes with advanced support to create, edit, play, convert and manage musical files with ABC text format support.