A project that tries to build an OS that mimics Windows and its functionality, without stealing code from it. It should be fully supportive for Windows-.based software. Truly open source, they are in an early Alpha, started a fund raiser (goal 50,000) and hope to have a working version in around two years. A german article that pointed me to this said that they have written 5 million lines of code so far - Windows XP, which has currently its last official MS Windows update being delivered, had around 40 million lines.
http://www.reactos.org/
Interesting for people using plenty of Windows software, but avoid Windows 8 and also do not accept of getting forced to go cloud after the next Windows version which is rumoured to be the last one MS will develop for system-installation.
I personally will avoid the cloud. If that means to go ReactOS or Linux, then so be it.