ReactOS - Open Source Windows
Now here is a wacky idea! Would that not make Bill Gates have sleepless nights, even though he no longer runs Microsoft. One such company and a set of brilliant entrepreneurs are doing just that at ReactOS. They might risk getting sued by Microsoft or some Unix folks in the coming months for mimicking some proprietary features or patent infringement, but if everyone keeps their cool then it is as wacky as it is brilliant. Here is why.
The operating system under consideration, ReactOS, is based on the earlier versions of Windows. Hence, it is not really directly competing with Vista or even XP. Hence, we wont be seeing it running on modern PCs anytime soon.
The PC world still has hundreds of millions of old PCs being used around the world dating back to the original Pentium machines when Microsoft's Windows 95 actually came out.