Complete DC n00b looking to get started

[miazmaticdotcom]

I have assorted Windows, Linux (x86 and PPC), and Mac machines standing around in here. Other than a few hours of my tinkering every day, they pretty much stand unused.

I wonder, how can I put them into a cluster, and once they are in said cluster, what applications can make good use of it?

(I am also planning to cluster a few hacked XboXes/PS2s. Xboxen are x86, FWIW.)