Mesen is a cross-platform libretro core for NES/Famicom built in C++. Older standalone versions are available elsewhere.
Website (https://www.mesen.ca)
Documentation
The latest version is available from the releases tab on GitHub. Arch Linux has a package on the AUR for Arch Linux. OpenSUSE Tumbleweed has a package on the OBS, however the repository it's in hasn't been updated in a long time.
See COMPILING.md
Mesen is available under the GPL V3 license. Full text here: http://www.gnu.org/licenses/gpl-3.0.en.html
Copyright (C) 2019 M. Bibaud
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.