Could it be the Firefox of Operating Systems?
Up until about this time last year it was though that Microsoft's dominance in the field of the web browser was indisputable. Along came a little web browser called Firefox, and for the first time ever a Microsoft product actually lost market share. While Linux has long been considered to be the challenger of Microsoft's monopoly in operating systems but has never gone beyond running servers in certain corporate environments. Greater ease of use and more software are the main reasons that Windows continues to dominate desktop computing.
There's a new operating system being developed by open source programmers called ReactOS www.reactos.org with the goal of being fully compatible with Windows programs and device drivers. Although it's still in it's early development, ReactOS can already be downloaded and tried out. One option is a 'Live CD', which is an .iso file that can be downloaded and burnt onto CD-R, which creates a CD that boots the computer, and the entire operating system loads from the CD. The other option is a full installation onto the computer's hard drive.
ReactOS is still in the pre-alpha stage so there is very little support for even the most common hardware. I had to dig out an old ps/2 mouse since my wireless USB mouse didn't work. ReactOS can only be installed on hard drives using the Fat 32 file system only. The NTFS File system is not yet supported and probably won't be for a while yet.
So far ReactOS is small and fast, using just a minute 30 megabytes when installed. Even booting from the Live CD, ReactOS boots up and is up and running in a fraction of the time that it takes a similarly equipped Windows PC. When ReactOS moves beyond initial development towards the first full official release, many of those who will benefit by having a new choice in Operating System will be those who can't afford to or just don't choose to upgrade to new hardware. As Windows get continually bogged down with more and more features that people don't use more and more will just not bother spending more money on an ever more expensive copy of Windows.
It will be interesting to see if the ReactOS developer community can meet their goals without infringing on Microsoft patents. If ReactOS does at one point come to challenge Microsoft then at the very least cater to PC users with basic needs with basic hardware which many would say that Microsoft has forsaken long time ago.