ReactOS is built from scratch and can run Windows binary files, at least in theory
ReactOS 0.3.16, a free open source operating system based on the best design principles found in the Windows NT architecture, is available for download.The current version of ReactOS, 0.3.19, has been in development for over a year, just like the previous build before it, and it incorporates architectural enhancements for the NT architecture.
“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,” notes the developer.
According to the developers of ReactOS, the main goal of the project is to provide an operating system which is binary compatible with Windows.
A complete list of updates and new features can be found in the official announcement. You can download this interesting operating system, ReactOS 0.3.16, from the official website.