ReactOS

ReactOS is an open-source OS aiming for binary compatibility with Windows applications and drivers. It reimplements the Windows NT architecture, letting many Windows executables run natively. Though still alpha, it includes a similar file explorer, control panel, and registry system. The project aspires to provide a free alternative for Windows software, especially older apps. Developers can test and debug Windows software in an open environment. ReactOS is not production-ready, but it showcases a unique approach to Windows compatibility in a free OS form.