LAM is a freeware implementation of the Message Passing Interface parallel programming API.
It is especially friendly to clusters. The program includes a persistent runtime environment for parallel jobs, support for all of MPI-1 (except MPI_Canceling sent messages), support for a good chunk of MPI-2 and support for MPI-IO (provided by the ROMIO package from Argonne National Labs).
LAM/MPI version 6.3.x is the current stable release. LAM/MPI 6.4.x is also available and includes support for Interoperable MPI.