ReactOS is a free and open source operating system that is Windows compatible and written from scratch. It is still in Alpha Testing but has made some advancements. It was made more stable via bug fixes and unit tests, and it has added some Windows Vista API calls to run some modern Windows software.
With Microsoft Windows 10 sucking so bad, there needs to be an alternative to it, ReactOS might be that alternative in 5 or 10 years from now. As of current it is not yet ready for prime time use.