The problem for us home users is there isn't really software designed out there to let us do this. You could probably find some brute force password cracking software, and I'm sure there are some other options as well. When doing this the only tasks that I'm aware of that work on these systems are those distributed programs like SETI and folding at home. Something that you can assign Computer 1 to do chunk 1, computer 2 to do chunk 2, etc. I don't think rendering like blender would work very well, or if it even supports working in this way.
This is different then just adding another core. The program needs to know which chunks are being worked on, and where they are on the network. This is different then just which core has the info. If its not network aware, then its not happening.
Doing for the sake of doing is one thing, but I think you'll find a big lack of software/tasks that you would care about.