Baseenv is my own collection of routines and programs that I have found valuable, particularly in working with distributed memory parallel computers. The most recent tarball is available. This package is being managed at bitbucket as a private project; email me if you are interested in access to the repository. Unfortunately, there is apparently no easy way to make the downloads available to everyone from bitbucket.
MPICH is a freely available implementation of the MPI Message Passing Interface standard.

Also available are sample programs and tutorials

The Portable, Extensible Toolkit for Scientific computing (PETSc) is a freely available package for solving large linear and nonlinear systems of equations. PETSc can be used on single workstations (or even laptops) or on parallel machines of all kinds.
Sowing is a collection of tools for developing portable programs. The major tool in this collection is doctext, a program for creating documentation from structured comments within source code.
Computer Science Department
University of Illinois Urbana-Champaign