ReactOS is a free open source operating system based on the best design principles found in the Windows NT architecture (Windows versions such as Windows XP, Windows 7, Windows Server 2012 are built on Windows NT architecture). Written completely from scratch, ReactOS is not a Linux based system, and shares none of the UNIX architecture.
The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows. This will allow your Windows applications and drivers to run as they would on your Windows system. Additionally, the look and feel of the Windows operating system is used, such that people accustomed to the familiar user interface of Windows would find using ReactOS straightforward. The ultimate goal of ReactOS is to allow you to use it as alternative to Windows without the need to change software you are used to.
ReactOS is an effort to create a Free and Open Source replacement for the Microsoft Windows NT family that is compatible with both applications and drivers. The NT architecture has always been highly flexible and powerful and its continued dominance in the computer industry means it is one of the most supported family of operating systems in existence, with its latest iteration being Windows 8.
As these days operating systems are little more than gateways to applications that users want to run, an open source NT implementation would allow users to continue using familiar programs in a familiar environment. The project seeks to embrace the strengths of the NT family while avoiding many of the configuration decisions that made older versions of Windows vulnerable and maintaining a lightweight environment so that a computer's resources can be dedicated to what really matters to the user, running their applications.
A little under a year has passed since the previous release and a significant amount of progress has been made. Some of the most significant include completion of the CSRSS rewrite and the first stages of a shell32 rewrite. 0.3.16 is in many ways a prelude to several new features that will provide a noticeable enhancement to user visible functionality.
A preview can be seen in the form of theme support, which while disabled by default can be turned on to demonstrate the Lautus theme developed by community member Maciej Janiszewki. Another user visible change is a new network card driver for the RTL8139, allowing ReactOS to support newer versions of QEMU out of the box.
And for those of you that have not heard of it yet, the project is running a Kickstarter campaign in the form of the Thorium Cloud Desktop. If you want to help the project raise the funds to hire multiple full time developers and bring ReactOS to a state where it can be used for day to day activities.
Download: ReactOS 0.3.16