Foss Music Notation
MuseScore is built by an open-source community.
from the MuseScore web site
MuseScore is a scorewriter for Windows, macOS, and Linux, comparable to Finale and Sibelius, supporting a wide variety of file formats and input methods. It is released as free and open-source software under the GNU General Public License. MuseScore is accompanied by a freemium mobile score viewer and playback app, and an online score sharing platform. The makers of MuseScore claim that it is the world's most popular notation software.
MuseScore was created as a fork of the MusE sequencer's codebase. At that time, MusE included notation capabilities and in 2002, Werner Schweer, one of the MusE developers, decided to remove notation support from MusE and fork the code into a stand-alone notation program.
The musescore.org website, created in 2008, and quickly showed a rapidly rising number of MuseScore downloads. By December 2008, the download rate was up to 15,000 per month.
After many, many years of Sibelius, I have converted all my hundreds of scores to using the open-source software MuseScore . It is now my go-to music notation program. - Curt
More information available on the MuseScore's web site.
Even more information, discussion and comments on this listing below.