MPI (http://www.mpi-forum.org/) is a popular standard for distributed-memory parallel programming. It offers both point-to-point message passing and group communication operations (broadcast, scatter/gather, etc).
Open MPI (http://www.open-mpi.org/) is an implementation of the MPI standard that combines technologies and resources from several other projects (FT-MPI, LA-MPI, LAM/MPI, and PACX-MPI) in order to build the best MPI library available.
The MLton MPI bindings provide an ML interface to a subset of the MPI 1.2 procedures for communication. It is based on the Ocaml MPI library by Xavier Leroy (http://pauillac.inria.fr/~xleroy/software.html#ocamlmpi). The mpi library has been tested with Open MPI and MPICH.