ReactOS is an advanced, free, and open-source operating system providing a ground-up implementation of Windows NT compatible operating system.
It aims to achieve complete binary compatibility with both applications and device drivers meant for NT and XP operating systems by using a similar architecture and providing a complete and equivalent public interface.
ReactOS is a great example of an almost complete working model of a Windows-like operating system available at this time. Consequently, working programmers will learn a great deal by studying the source code and even participating in development.
In short, ReactOS is aiming to run your applications and use your hardware, a free operating system for everyone.