As of Nov 2007 it's the fastest computer in the world.
IBM BlueGene/L
Location: Lawrence Livermore National Laboratory
Processor Type: PowerPC 440 700 MHz (2.8 GFlops)
Number of processors: 212,992
Main Memory: 73,728 GB
Disk space: 1.9 petabytes
Peak Speed: 596 teraFLOPS
Check out the slanted front panels that are needed for cooling.
This is just what you need to dash off some predictive, fully integrated 3D nuclear weapons calculations. Or maybe to answer those nagging plutonium aging questions that keep you up nights.
More info here