This has been around for a long time, but I'd never heard of it: ReactOS is an open-source re-implementation of Windows. I guess it's possible to be fanatically devoted to both Windows and open source. As mammoth a task as this sounds, it seems they are making progress. Although they've been at it for about ten years, they have screenshots of working code, and an active subversion repository (they're working on DirectX support now).
I wonder what the future will hold for ReactOS. Microsoft will continue to build Windows, widening the gap between what Windows and ReactOS are, though if the reaction to Vista is any indication, perhaps XP and its ReactOS clone will be considered the golden age of Windows. At the same time, anti-Microsoft sentiment will continue to build among the open source community, either in the pro-Linux or pro-Apple flavor.