The supercomputers on the list are used for a wide range of purposes from basic research to weather forecasting and military applications. The TOP500 list is published every six months and is based on the Linpack performance test, which measures how many floating point operations per second (flops) a machine is capable of. All the performances in the list are quoted in teraflops, where one teraflop is a trillion flops (or 1012 flops).

Blue Gene/L currently contains 16,384 processors, but this number is due to increase by a factor of four to 65,536, which should make it capable of 360 teraflops. Currently located at an IBM facility in Minnesota, Blue Gene/L will be moved to the Lawrence Livermore National Laboratory in California where it will mainly be used to study how nuclear weapons age as part of the US stockpile stewardship programme.

"BG/L will reduce the time-to-solution for many computational problems, allowing DOE scientists to explore larger, longer and more complex problems than ever before," said Spencer Abraham, head of the DOE. "For example, a heroic 30-day calculation on what was the number three supercomputer in summer 2003 would now be completed on BG/L in about three days."

NASA's Columbia machine, which was built by Silicon Graphics, will be used for hurricane predictions, studies of global warming and astrophysics research. Meanwhile, the Earth Simulator - an NEC machine that is based at the Earth Simulator Center in Yokohama - is used for climate modelling and simulating seismic activity. It was number one in the TOP500 for two-and-a-half years before being overtaken by Blue Gene/L and Columbia.

The highest ranked European machine is an IBM-built MareNostrum cluster at the Barcelona Supercomputer Center in Spain, which is capable of 20.53 teraflops.