ReactOS

ReactOS is an open source computer operating system intended to be binary compatible with application software and device drivers made for Microsoft Windows NT versions 5.x and up (Windows 2000 and its successors). A spin-off of a previous attempt to clone Windows 95, development started in early 1998, and has continued with the incremental addition of features already found in Windows.

ReactOS is primarily written in C, with some elements, such as ReactOS Explorer, written in C++. The project has been ported to the ARM and AMD64 processor architectures, and partially implements Windows API functionality. The latter is assisted by including parts from the Wine compatibility layer for Unix-like operating systems, but other functionality is implemented by the developers themselves. However, progress has been hampered by a lack of developers with the relevant skill-sets.

An extensive code audit is in place to protect against legal problems, such that implementation of the Windows API is only done by means of a complete clean room reverse engineering process. This has been in place following claims made in 2006 by a former developer that the project contains disassembled assembly code from Windows and a generally cautious approach. The allegation and audit has found no compromised code and has had no adverse legal consequences to the project.

ReactOS has been noted for its information on undocumented Windows APIs and more generally as a potential open-source drop-in replacement for Windows. "The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows ... 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 remove Windows® and install ReactOS without the end user noticing the change."