MPICH is a portable implementation of the MPI (Message Passing
Interface) standard. The goal of MPI, simply stated, is to develop a
widely used standard for writing message-passing programs. As such the
interface should establish a practical, portable, efficient, and
flexible standard for message passing. Included are a full set of
documents and parallel programming examples.

Note: with the exception of users requiring the communication of heterogeneous
data, the authors strongly encourage everyone to consider switching to MPICH2.

WWW: http://www.mcs.anl.gov/mpi/mpich1/