I recently built a Raspberry Pi cluster just to see how it's done. I have no idea how to use a super computer, so I built a web server cluster instead. I used four Raspberry Pi nodes, and I'm using a PC as a load balancer to distribute HTTP requests to the Pies. I plan to replace the PC with another Raspberry Pi at some point.
I used Apache on each worker node, and I installed Apache with proxy, proxy_http and proxy_balancer modules on the load balancer. The load balancer has two ethernet sockets. One is connected to my home router, and the other is connected to an ethernet switch. The worker nodes are also connected to the switch.
I'm using a CMS that I wrote myself and it has a simple page caching system, so performance is reasonably fast. Even without page caching, page load times are still quite good as there's no SQL database in my site.
You can read more about the set up here: http://raspberrywebserver.com/raspberrypicluster/raspberry-pi-cluster.html