Sun has released their Grid
Sun has released their Grid Engine distributed computing project as open source. I'm not sure I'd ever have a practical use for this software, but it's very cool. Essentially, it lets you spread computing tasks over multiple machines, taking resources from systems that are lightly used. I've always heard that ILM, Pixar, and the other big rendering companies do something like this -- when people go home for the night, their workstation becomes part of the render farm. Cool.