as for major OSs, let me just point out that *BSD and linux are NOT the same. and at a push, GNU and linux are not the same either. also, i can say too that there are probably linux systems that are more different from each other than linux is to, say, BSD, so the question is misleading. while linux, bsd et al are related, they do add up to at least three or four totally distinct operating systems, all major, all for desktops.
also, there's the likes of SunOS, Solaris, TOPS10, AIX, HP-UX, UNIX, and i think there are five major BSD versions. none of those have any linux in them, and apart from each of them probably having some GNU tools installed on them (but not bundled probably) they are all unique from each other.
Also, does OS/2 still count as a major OS? i might mention FreeDOS too, which is a much better PC operating system than any other DOS i have ever seen.