Although it changes frequently see:
http://en.wikipedia.org/wiki/TOP500
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It is the "IBM Roadrunner". It is located at Los Alamos National Laboratory in New Mexico, USA. It has 103.6 TeraBytes of RAM, 12,960 IBM PowerXCell 8i CPUs, 6,480 AMD Opteron dual-core processors and it costs US$133M.
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Tour of Duty - 1987 Roadrunner 1-11 was released on: