ReactOS is an advanced free open source operating system providing a ground-up implementation of a Microsoft Windows XP compatible operating system. ReactOS 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 start menu
ReactOS running Unreal Tournament
ReactOS is closely related to the Wine project (the Linux application that can run Windows application) and can therefore run most of the Windows applications, that work with Wine. Installing program in ReactOS is simple point and click, which is ten times easier than getting applications to run on Wine. If you use Linux and then Wine to run Windows application, why not use ReactOS instead?
A detailed review of ReactOS