ReactOS

ReactOS is a free implementation of the NT architecture and the Win32 API. It's not based on Unix or nor shares anything with Unix. It's goal to reach program and driver compatibility with Windows. Currently, the support level is Windows Server 2003 SP2.

Current state
It's still considered as alpha software and only recommended for installation and testing purposes. Installation, hardware detecting, booting, running programs are plagued with issues. It's recommended to try ReactOS in a virtual machine. ReactOS works best in QEMU, however, it can run on other hypervisors. It's possible to install ReactOS on a physical machine, however, this configuration is still quite risky.

ReactOS 0.4.0 is rumored to be released in Holidays 2015, it's possible to download a nightly build from the ReactOS website though. It will bring many new features, the major ones being a new Explorer and Application Manager. ReactOS 0.4 will also improve sound, Wi-Fi and USB support.