Starting this to discuss HPC's. This is used now to solve lots of problems in science, industry, finance, weather using not just huge massive computers but computers netowked together to solve problems using parallel computing.
An application area related to a posted forum topic is computer 3d graphics - crowd simulation. This takes a lot of time, sometimes days/weeks on a high-end workstation to render a high quality picture especially with advanced ray tracing, but this can be reduced to hours using even a couple of computers working together. For my 3ds max crowd simulation, l rendered the resulting graphics and avi animation file in a couple of hours using the backburner networking software and 3 machines l had on my LAN (local area network).
All major movie production firms have or source out their own cluster power called render farms.
Google does the same. Note, google choose to cost-effectively build their cluster using lots of low powered pentium boxes instead of expensive top of the range machines.
For those interested in adding their personal computer's computing power towards solving medical issues pertaining to africa and the world, see: http://www.worldcommunitygrid.org and l'll suggest joining team "african3". This is also known as BOINC.
Open source software such as ROCKS (based on the well known stable linux distro Centos ) enables you to build a cluster very easily - google for their website.
Anyone needing help on such projects please. feel free to contact me.

Making progress: http://stats.kwsn.net/team_history.php?proj=wcg&teamid=16410

Currently running on a Rock cluster of 4 linux boxes, 2 xp machines and one win 2003 box