Open Terracotta is a cool open source clustering technology for Java that isn't yet supported on OS X. Good news, though, is that it already works.
There are some simple instructions in this blog post.
[robg adds: As submitted, this hint referenced a URL that no longer resolves, so I used Google to find the above-linked blog post. If the instructions there are not what the original anonymous poster intended, I apologize, but I had no way of contacting them to confirm the URL, as they didn't include their email address.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061204101629841