Remember, thats only 500 American Corporate Users. There's no way of knowing the real answer because it depends on who and where you ask. I have been in this business for 40 years. I've worked on large Univac and IBM systems, minicomputers (like Data General and DEC), and with microprocessors since their introduction in 1972 (Hey, even before Gates got into them). I have literally designed, built, and programmed systems on over a thousand different machines over the years. In America, Windows is the hands down winner for Desktop systems. This is primarily because of Microsofts marketing in the OEM manufacturing arena. They were brutal, but in the end virtually every manufacturer of any size must pre-install Windows on their Desktops before they go out the door. If they don't, they won't be allowed to do business with Microsoft. I know! I was an engineer for Unisys when Microsoft drove them completely out of the PC Manufacturing business! The server business is a little different. Servers are built to support multiple different operating systems. Some servers are shipped without any operating system at all. The differences between UNIX and Linux are virtually a moot point now. Ever since Caldera inherited the rights to the UNIX name (along with the remains of Unix Systems Labs (USL)), UNIX and Linux have been merging constantly closer together. What were the last (traditional?) UNIX systems (SVR and UnixWare based) like HP/UX and Solaris, are today primarily based on Linux kernels. Those that were of the Berkley UNIX flavor, are today generaly based on Free BSD, Net BSD, and Open BSD. Even the Macintosh, running OS/X is built on a Free BSD kernel. So is Darwin, the open-source verson of OS/X. The World Wide Web (www) was built and primarily still runs on Unix and Linux based servers. Much of the government systems (except desktops), run on UNIX and Linux. Unix and Linux are a given in the University world. Real Time and Embedded systems are a whole other world. There are more "embedded" microprocessors manufactured every year than all of the PC's in the world put together! Many use proprietary sytems for development, but Unix and Linux are the hands down winners here for "cross platform" development. When you look outside of the US, the international community has a number of other operating systems that are used to some extent. Windows, Unix and Linux however, are the dominant players. If we were just counting numbers, then China would be a major factor. Microsoft has been tr