Although it has started around 1996, today few know about the existence of ReactOS project. It's an effort to create a Free Software replacement for Microsoft Windows that is compatible with existing hardware and software. In other words, a Free Windows.
ReactOS is a free and open source operating system designed from scratch based on Windows NT design architecture aiming to provide full support for existing Windows applications and drivers, and to be an alternative to consumer edition of Windows like XP, and Vista and to server edition like Windows server 2003.
By now, some of you may ask why implement such as thing? I say why not? but if you insist on knowing why then read this!
Actually, ReactOS is still in Alpha stage and a Beta version may appear next year offering a more stable version for day to day use. It's available for download as an ISO file and as preloaded virtual machine for QEMU and VMWare. But if you are not so adventures you can have a look at the screenshots.
Personally, I tested it in Sun xVM VirtualBox and found it to be very promising though many things are still lacking like Audio and Network Sharing support.
It worth noting that there is also a project named FreeDOS aiming to replace the old MS-DOS. It has already implemented a better DOS. Yes, I hear you saying "Wake up! DOS days are over man!". Well, I already know that. But wait, have you ever bought a new PC without an operating system? Chances are you would either receive a FreeDOS CD or it has already been installed by default.
In other words, think about the possibility of shipping ReactOS instead of Windows by 2014, when Microsoft will end XP support leaving a huge PC base stuck with XP for one reason or the other.
More realistically, though by 2014 the IT landscape will be different from now, we will have this idea stuck in our mind that when companies leave their old products, open source will always take care of that. And in one way or another that will always be of a positive impact on the Free and Open Source Movement.