RetrOS-32 is a 32bit operating system written from scratch, with graphics, multitasking and networking capabilities. The kernel is written in C and assembly, while the userspace applications are written in C++, using Make for compilation, all licensed under the MIT license.
It runs on Qemu, of course, but a variety of real hardware is also supported, which is pretty cool and relatively unique for a small hobby project like this. The UI is delightfully retro – as the name obviously implies – and it comes with a set of basic applications, as well as games like Wolfenstein 3D.