Source: http://www.nsa.gov:8080
The National Security Agency/Central Security Service is responsible for the centralized coordination, direction, and performance of highly specialized technical functions in support of U.S. Government activities to protect U.S. communications and produce foreign intelligence information. The National Security Agency (NSA) was established by Presidential directive in 1952 as a separately organized agency within the Department of Defense under the direction, authority, and control of the Secretary of Defense, who acts as Executive Agent of the U.S. government for the production of communications intelligence (COMINT) information.
Detailed information about the computational resources at NSA are not directly available. However, it is believed that the largest Cray T3D ever built, a 1024 processor machine, is installed there. Evidence for the existence of this machine are NAS Parallel Benchmarks results, which were run on a 1024 T3D before this machine was installed at a "classified" site. The count of actual machines at this site may be inaccurate, but NSA is definitely a TOP10 site as far as supercomputing is concerned.