In 1993/1994, at NASA’s Goddard Space Flight Center, Donald Becker and Thomas Sterling designed a Commodity Off The Shelf (COTS) supercomputer: Beowulf. Since they couldn’t afford a traditional supercomputer, they built a cluster computer made up of 16 Intel 486 DX4 processors, which were connected by channel bonded Ethernet. This Beowulf supercomputer was an instant success.

Source: ​Linux totally dominates supercomputers | ZDNet

Linux first appeared on the Top500 in 1998. Before Linux took the lead, Unix was supercomputing’s top operating system. Since 2003, the Top500 was on its way to Linux domination. By 2004, Linux had taken the lead for good.