Beowulf: a class of computer systems using free software and commodity hardware to create supercomputers
a) Beowulf Project Homepage

MPI : The Message Passing Interface is an open standard for allowing one to create distributed programs over a network.
a) MPI Homepage
b) MPI Forum
c) Comp.parallel.mpi - Usenet newsgroup for discussion of MPI

PVM: PVM (“parallel virtual machine”) is one of the most popular software systems for creating distributed-processing applications.
a) PVM Homepage
b) Comp.parallel.pvm - Usenet newsgroup for discussion of PVM
c) PVM port to Windows (ORNL)
d) WPVM - another port of PVM to Windows (defunct)

Sample Applications and Libraries
a) SETI - Search for Extraterrestial Intelligence
b) FinDer - uses PVM to price mortgage-backed securities
c) Netlib - repository of mathematical software
d) Top 500  Supercomputer Sites - some of these are clusters