Linux runs the majority of the world's servers — for free

In 1991, Finnish student Linus Torvalds posted a message to a newsgroup announcing a 'free operating system' he was building as a hobby — 'just for fun,' he said. Linux, built collaboratively by thousands of volunteers under an open-source licence, now runs over 96% of the world's top one million web servers, every Android phone, and the overwhelming majority of cloud infrastructure. The Linux kernel alone contains over 30 million lines of code contributed by developers at companies including Google, Intel, and Microsoft.

Source: Torvalds & Diamond, Just for Fun: The Story of an Accidental Revolutionary (2001)