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People once (and still do) use something else and turned to Linux because they were being overcharged. So is there anything else that is free? Yes, there are other operating systems that are free. There are various ones, but the one I find notable is Solaris. Why? Because it has been widely used in mission-critical applications since the 80s and has had a large focus on scalability. Solaris is a Unix operating system that found itself abandoned by many for Linux. The main underlying problem was price. Well, after some controversial internal politics and legal maneuvering within Sun they released Solaris as free in 2005 and even placed development in a community-run project named Opensolaris. Problem solved. Thanks to Linux.
Some of you may be concerned with speed or compatibility but Solaris has done much to address this since it became free with numerous world records in performance and a bigger compatibility list than any single distribution of Linux.
Now why not AIX or HP-UX?. Two reasons really. They aren't free. They don't run on x86 architecture, which is by far the majority of the market.
Why not Windows? Let's go with it's not free, although there are many, many other reasons.
There is also hardware backing for Solaris from IBM, HP, Dell, Fujitsu, and of course Sun/Oracle. So will Linux end up being just a price check for Unix?