@What for ?
"So, are these machines trying to inverse a 10 x 10 matrix, or compute pi to a billion places or what ? I can understand the NSA trying to decrpt millions upon millions of messages simultaneously but what on earth are these machines used for in the real world... and why are sales banned to Cuba, Iran and North Korea ?"
Think of manipulating 30000x30000 matrices. Many simulations are still pretty rough approximations of the physical world. That's why you need to detonate a nuke before you can be confident it does what you want it to do, even today.
Other big numbercrunching problems are in drug desgin and medical research, simulating the interactions of a large molecule with another one. These ones still defy the largest contemporary computers in many instances.
Or.. simulate a galay of 100 million stars. That yields in 100E6*100E6 force vectors to be computed and added up. That's 1E16 operations per iteration of your simulation. The fastest CPU does just 1E12 operations/sec.
I bet the nanotech people also can make good use of powerful simulations to compute what a certain surface does to another surface or molecule.
Obviously, these systems can be used for all sorts of military R&D and consequentially are export-controlled.
Iran and the other evildoers can nowadays build pretty powerful machines just out of a bunch of beige boyes and 10Gbit ethernet cards and the latest switches, though. There was even a story claiming the evilers would be using masses of powerful PlayStation machines to that end.
TUX as most supercomputers are now Linux.