Open source Music notation for Windows

Denemo

Denemo is a free software music notation editor for GNU/Linux, Mac OSX and MS Windows that lets you rapidly enter notation for typesetting via the LilyPond music engraver. You can compose, transcribe, arrange, listen to the music and much more. (GPL)

Linux - Windows

Impro-Visor

Impro-Visor is a music notation program designed to help jazz musicians compose and hear solos similar to ones that might be improvised. The objective is to improve understanding of solo construction and tune chord changes. (GPL)

Cross-platform

LilyPond

LilyPond is an automated engraving system. (GPL)

Linux - FreeBSD - Windows

MuseScore

MuseScore is a free cross platform WYSIWYG music notation program. (GPL)

Linux - Windows

TuxGuitar

TuxGuitar is a Multitrack tablature editor and player. (LGPL)

Linux - FreeBSD - Windows - MAC OS X

5 applications