Implementations that are currently available for various platforms include:

Maintained implementations

• MPICH: A popular, free and open source (FOSS), portable implementation from Argonne National Lab. Fully supports MPI-3.
• Open MPI: Another FOSS implementation, with an emphasis on portability and simple installation and configuration. Fully supports MPI-3.
• Intel MPI: A vendor-supported version of MPICH from Intel. Fully supports MPI-3.
• MVAPICH2: MPICH derivative from Ohio State University with optimizations primarily for InfiniBand, but also for Omni-Path, iWARP, and other RDMA-enabled interconnects (including GPUs, see MVAPICH2-GDR). Another variant (MVAPICH2-X) is designed with exascale computing systems in mind, in which both MPI and OpenSHMEM communications may be important. Fully supports MPI-3.
• IBM Spectrum MPI: Successor to IBM Platform MPI, a vendor-supported MPICH from IBM.
• MS MPI: A vendor-supported version of MPICH2 from Microsoft, now part of Microsoft HPC Pack (Windows HPC Server).
• MPI/Pro: A vendor-supplied implementation from RunTime Computing Solutions (originally developed by MSTI = MPI Software Technologies, Inc., and later sold to Verari Systems).

Deprecated or defunct implementations

• HP-MPI: A vendor-supplied implementation from Hewlett-Packard. In 2009 it was rolled into Platform MPI (formerly Scali MPI), which ultimately became IBM Platform MPI (see above).
• LAM/MPI: Open MPI's forerunner from Indiana University (maintenance mode only).