Parallel Computing Links:

Beowulf Project  -- Information on using multiple ethernet cards to form high-speed parallel Linux clusters.
A Current PVM RPM is maintained by the Beowulf project.
PVM Home Page
MPI Home Page
PVM Information from JICS  -- Lots of good PVM links including the best beginner's guide I've seen.
Linux Parallel Processing HOWTO (old)  -- Excellent Parallel Processing Hardware/Software primer.
Linux Parallel Processing HOWTO (new)  -- Excellent Parallel Processing Hardware/Software primer.
Highly-parallel Integrated Virtual Environment  HIVE -- Another NASA cluster site (Beowulf is at NASA too).
NHSE Catalog of Software Tools  -- Exactly as it says.
PVM and MPI: A Comparison of Features  -- Good comparison of the two.

Beginner's Guides:

A Beginner's Guide to PVM (From JICS)  -- Very Good and Easy to Understand.
Getting Started with POSIX Threads  -- Maybe even too simple? Read this first.
How To Program With Threads  -- More detailed but also good.

Example Code:

PVM Matrix Multiply: Master
PVM Matrix Multiply: Worker