ReactOS

ReactOS is an open-source operating system for x86/x64 PC intended to be binary-compatible with computer programs and device drivers made for Windows Server 2003.

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

The ReactOS and the Wine projects share the goal to run binary Windows software natively and can share therefore many dependencies and development. ReactOS uses portions of the Wine project so that it can benefit from Wine's progress in implementing the Win32 API. While Wine's NTDLL, USER32, KERNEL32, GDI32 and ADVAPI32 components cannot be used directly by ReactOS due to architectural differences, code snippets of them and other parts can be shared between both projects. The kernel is developed by ReactOS separately as Wine relies here on existing unixoid kernels.

Separately, the experimental Arwinss branch was created as an alternative means to improve USER32 and GDI32 support through an alternative implementation of the Win32 API. Whereas ReactOS's original Win32 subsystem was closely modeled after its equivalent in Windows, Arwinss combines the architecture of that subsystem with the corresponding implementation in Wine. To this end, Arwinss uses Wine's GDI32 and USER32 libraries with few changes to take fuller advantage of Wine's existing software compatibility. Arwinss also allows the user to optionally use a remote X server instead of a local display.

Others

The Tango Desktop Project initiative provides open-source design guidelines and ressources (as icons) for applications on desktop environments. FreeType is an open-source software development library, used to render text on to bitmaps and provides support for other font-related operations. The KernelEx project is an Windows-API extension and compatibility layer project, which provides open-source implementations of some Windows-APIs. Other contributing projects are MinGW, SYSLINUX, adns, ICU, GraphApp, Ext2, GNU FreeFont, DejaVu fonts, Liberation fonts.

Now in 2015-16

As of now the project is in pre-beta stage with its current new preview release of 0.4 RC1 version which is available free to download on its website

Visit: ReactOS

Source: Wiki