Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
If you have a couple of old Linux boxes sitting around, then you've got the makings of a supercomputer. Dust them off, install Secure Shell (SSH) and Parallel Virtual Machine (PVM), and start your complex algorithms.
All right, it's not quite as simple as that. PVM handles only the messaging between the machines. You must write your own programs to actually do anything.
First, network your PCs and set up NFS on each. I'm not going to go into detail because most Linux distributions take care of everything for you. With Debian, for example, simply connect a cable between your new PC and your network switch, stick in your installation CD, switch the PC on, and follow the prompts. If you need more information, take a look at the Linux.com how-tos on networking and NFS.
Now you can start setting up your PCs as a single supercomputer.