LAM/MPI

LAM/MPI is a high-quality open-source implementation of the Message Passing Interface specification, including all of MPI-1.2 and much of MPI-2. Intended for production as well as research use, LAM/MPI includes a rich set of features for system admin

PVM

PVM (Parallel Virtual Machine) is a software package that permits a heterogeneous collection of Unix and/or Windows computers hooked together by a network to be used as a single large parallel computer. Thus large computational problems can be solved

MPICH

MPICH is a freely available, portable implementation of MPI, the Standard for message-passing libraries.

Open MPI

Open MPI is a project combining 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. A completely new MPI-2 compliant implementation, Open MPI offers advantag

Java Parallel Processing Framework

The Java Parallel Processing Framework is a suite of software libraries and tools providing convenient ways to parallelize CPU-intensive processing. It is written in the Java programming language and is platform-independant.

MPI Forum (MPI-1 and MPI-2 specifications documents)

You can read the MPI specification in these documents.