We present MPWide, a platform independent communication library for
performing message passing between computers. Our library allows coupling of
several local MPI applications through a long distance network and is
specifically optimized for such communications. The implementation is
deliberately kept light-weight, platform independent and the library can be
installed and used without administrative privileges. The only requirements are
a C++ compiler and at least one open port to a wide area network on each site.
In this paper we present the library, describe the user interface, present
performance tests and apply MPWide in a large scale cosmological N-body
simulation on a network of two computers, one in Amsterdam and the other in
Tokyo.