Comment by zigzag312

https://github.com/MonoGame/MonoGame (runs even on consoles, which is rare for open source game frameworks and engines)

https://github.com/SubtitleEdit/subtitleedit

https://github.com/jellyfin/jellyfin

https://github.com/AvaloniaUI/Avalonia

https://github.com/bitwarden/server

https://github.com/unoplatform/uno

https://github.com/files-community/Files

https://github.com/NickeManarin/ScreenToGif

https://github.com/DevToys-app/DevToys

https://github.com/d2phap/ImageGlass

https://github.com/ShareX/ShareX

https://github.com/duplicati/duplicati

https://github.com/OpenRA/OpenRA

https://en.wikipedia.org/wiki/Ryujinx

https://sourceforge.net/projects/keepass/

https://github.com/orchardcms

https://github.com/piranhacms/piranha.core

https://github.com/umbraco/Umbraco-CMS