From a VentureWire article:

On Thursday [nov 5,2004], the Department of Energy claimed a
supercomputer being developed to analyze the mation's
nuclear stockpile achieved a sustained performance of
70.72 trillion calculations per second, becoming the
world's fastest computer. The machine, built by IBM at
a cost of roughly $100 million, runs on Linux, an
open-source operating system that is becoming a key
ingredient to high-powered, low-cost computing. But as
IBM presses ahead with its expensive Blue Gene
initiative, a trend is taking shape in the
supercomputing space.