ReactOS aka React Operating System is a binary-compatible open source remake of Microsoft Windows. This is the 0.3.11 install CD, which came out in December 2009, and even after ten years of development it is still in alpha stage. ReactOS is mostly governed by GPL, so it is free to use and distribute, but encumbrance exists, due to the project's nature, although proper functionality is achieved by clean-room reverse-engineering. Go to the ReactOS website for much more information, installation guides and a wiki, plus more downloads, such as pre-installed VMWare and VirtualBox virtual machines, a LiveCD and full source code.

It would be perhaps important to start by saying what ReactOS -isn't-. It is not another wrapper built on Linux, like WINE. It does not attempt or plan to compete with WINE; in fact, the user-mode part of ReactOS is almost entirely WINE-based and our two teams have cooperated closely in the past. ReactOS is also not "yet another OS". It does not attempt to be a third player, like SkyOS or any other alternative OS out there. People are not meant to uninstall Linux and use ReactOS instead; ReactOS is a replacement for Windows users who want a Windows replacement that behaves just like Windows.

While ReactOS' source availability and current usage is mostly intended for programmers to expand and improve on, you don't need to be a rocket scientist to use it, and in its final state will be a consumer friendly OS. If you've used Windows before, you'll find yourself in a familiar environment with ReactOS. The learning curve, if any, should be minimal, since ReactOS duplicates many of the Windows graphical environment applets, control panels and dialogs (Windows 2000/XP/2003).

Focus at present is on developing ReactOS to a stage where it is capable of running most Windows drivers and applications "out of the box." This does not mean that ReactOS will stop there, however; as new features are added to Windows, our developers be striving to incorporate such new features in future versions of ReactOS. Once the compatibility level has been reached, we may even choose to improve ReactOS beyond its Windows roots. These changes would be external from the main OS, and totally compatible with Windows, but they would add an extra advantage. For example, one of our goals is to have built-in POSIX compatibility, much like Windows Services for Unix. It could also be as simple as natively supporting multiple desktops however, instead of requiring a powertoy. 

A lot of people in popular discussion forums keep asking "why should I use ReactOS" or "why would someone need ReactOS" or "why not help develop Wine instead?" or "why not use Linux with Wine?".

We have an answer for all these questions, but it's not some simple magic word. Let's name a few key issues here:

    * There are plenty of *nix operating systems out there, this is very good. However they have different targeting (they perfectly fit server market, but desktop still isn't conquered, and several factors work against most Windows alternatives out today).
    * There is currently no operating system which implements the kernel architecture design of MS Windows NT family (GNU/Linux is the best for comparison here: Linux was started as “clone” of Minix and Unix (eventually going on to be a Unix replacement), and ReactOS was started as “clone” of Windows NT).
    * Linux+Wine is never going to be a complete replacement for a full Windows system. It's not only because Linux (despite there are some really user-friendly Linux distros out there), and not only because many users might find a transition to Linux/BSD difficult, but it's due to design and implementation decisions of Linux and Wine architectures, which prevent 100% compatibility.
    * Even though Linux supports many types of hardware, Windows is still the dominant platform for device manufactorers. There are attempts to overcome this situation (like NDIS Wrapper for NT network card drivers, there are rumours about supporting NT video drivers, Captive NTFS for NT filesystem support), but ReactOS solves them from the first day by its design – be compatible with existing drivers and existing applications.
    * There are many people who do not like how *nix systems behave or dislikes the conventions used. For them, Linux, BSD, and Mac OS X are not options, even before application compatibility and hardware support come into play. An operating system should give the consumers what they want instead of demanding the consumer conform. Even with WINE, you are still running an operating system that behaves quite differently from Windows, at a user and system level.
    * Backwards compatibility. This is something vital for many people and companies, but the development philosophy of Linux and the GNU project do not consider it a priority. The Windows family has always went out of its way to ensure a stable API and backwards compatibility. By its design, ReactOS will also follow the philosophy of backwards compatibility with existing and future applications designed for the Windows NT family.
    * There are no plans for Windows to become released under a GPL-compatible license (at least, ReactOS team is not aware of them).

Finally, ReactOS offers a third alternative, for people who are fed up with Microsoft's policies but do not want to give up the familiar environment, architectural design, and millions of existing software applications and thousands of hardware drivers.
 
http://www.reactos.org/