- Musescore.com is run by Muse Group, formerly Ultimate Guitar. This is the sheet music sharing site which has a few controversies with downloading user scores, although this has improved a bit recently.
- Musescore.org is the GPLv3 notation software which they've announced the release for.
Muse is very good at keeping these two parts separated from each other. For example, the new "Muse Sounds" is installed via the closed-source "Muse Hub" (which economically makes sense as providing high-quality samples would be a fruitful business opportunity in the future) through a shared library.
Personally I think that this is a nice balance between maintaining the open-source software and providing features that practically only work with commercial backing - the reason that this could occur so quickly is because it reuses the playback engine and samples from StaffPad (https://www.staffpad.net), one of Muse's acquisitions.