If you were to factor in phones and hand-held devices, there are actually a lot more operating systems in widespread consumer use. Obviously, the iPhone uses a version of Mac OSX, and there are some Windows-based phones too, but many other phones use other OSes. Google’s Android OS is a type of Linux (and there are rumors that they may extend this to PCs at some point).
There are consumer desktop, notebook and netbook PCs that run some flavor of Linux. The One Laptop Per Child XO laptop uses a version of Linux with a special user interface designed for children, called Sugar.
Also, Linux is popularly used on Linux Netbooks, though it’s not as prevalent as Windows XP. Finally, Xandros desktop (formerly Linspire, formerly Lindows) is a relatively easy to use and full featured version of Linux that one could use in place of Windows or Mac for many home uses (email, web, basic documents).
At this point, however, Mac OS X is designed in a way that far surpasses other OSes in terms of user experience. And, Windows is dominant in the corporate workplace to the point that the majority of computer users are comfortable with it. So, it is a bit like a 2-party system in that, whatever other parties are out there, most people just don’t pay a lot of attention to them.
