Beowulf is a general name for clustering low-cost computers such as PCs to create inexpensive "supercomputing" capability. Many people use the Debian operating system on Beowulf clusters for several reasons, from the excellent system management tools to package availability.
There is a Debian Beowulf page at http://www.debian.org/ports/beowulf/
There is also a debian-beowulf email list, with archives at http://lists.debian.org/debian-beowulf/
Beginners might be interested in how to StartaBeowulf
or take a look at: http://www.sintef.no/static/AM/vskoler/2001/notes/building_notes.pdf
Many Beowulf packages use UpdateCluster to simplify maintenance of their machines list
- Whether for Beowulf-style computations or just a network of workstations
Many parallel programs use MessagePassingInterface (MPI)
Many others use ParallelVirtualMachine (PVM)
http://wiki.debian.org/FAI is a tool for fully automatic installations of debian, and has been used for setting up many clusters.
There is a general Debian cluster how to that includes tutorials on many different aspects of Debian cluster setup and configuration at http://debianclusters.org/