In all fairness, BeOS is dead, XWindows is not an operating system but a GUI used mostly in *nix environments, GEOS, GEM and OS/2 are effectively dead, DEC had a flavor of Unix but I'm not sure whether that exists anymore. In current OS's, there's Windows, MacOS (FreeBSD Unix based) and all the various incarnations of Linux but that's about it for the consumer desktop as far as I know.
I still use Win2K on most of my machines, though I did put a recent version of Ubuntu Linux on one of my laptops to play with and I was shocked at how far it's come in the last few years. It still has a few rough edges but it's shaping up to be a very usable operating system and definitly something I'm interested in seeing after a couple more years of polish. If someone can come up with a solid unified configuration panel, settle on a standard sound driver interface and get the Windows emulator rock solid so it supports everything MS might have some real competition. Of course I don't really see it as a fight anyway, nothing is forcing me to use any operating system in particular, so I just use those which are most appropriate for what I'm doing with each particular computer I'm doing it on. Usually the choice comes down to what applications I need to run and what specific hardware is best supported.