HelenOS[1] is in C and has a GUI. I don’t know how many people participated over its (quite extensive) history. Axle[2] is a one-man project with a GUI that th

HelenOS[1] is in C and has a GUI. I don’t know how many people participated over its (quite extensive) history. Axle[2] is a one-man project with a GUI that the author has been gradually transitioning from C to Rust. Among C++ projects, I think Essence[3] also merits a mention. [1] http://www.helenos.org/ http://www.helenos.org/ [2] https://github.com/codyd51/axle https://github.com/codyd51/axle [3] https://gitlab.com/nakst/essence https://gitlab.com/nakst/essence