Linux
Linux was originally developed for personal computers.   Linux is the leading operating system (OS) on servers  such as mainframe computers, and the only OS used on supercomputers (at least on the TOP500 list, since November 2017). It is used by around 2.3% of desktop computers. The Chromebook, which runs Chrome OS based on the Linux kernel, dominates the US K–12 education market and represents nearly 20% of sub-$300 notebook sales in the U.S.