ReactOS

ReactOS is a free implementation of NT architecture an Win32 API. It's not based on Unix or nor shares anything with Unix. It's goal to be reach program and driver compatibility with Windows. Currently, the support level is Windows Server 2003 SP1.

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 VM (especially in QEMU, VirtualBox or VMware, MS Virtual PC 2007 has issues). You can still try it on real hardware, but you can easily encounter with an error that corrupts the entire system.

Note: Installing guest additions are recommended.

Upcoming versions
ReactOS 0.4.0 will be the next version and mark last series of alpha stage. It will bring new explorer and Applications Manager, will be more compatible with sound/Wi-Fi/USB.

It's rumored that 0.4.0 will come in Summer 2015.

MORE WILL COME