From the developers website:
"Ever thought that all that comment spam on your WordPress blog is useless and annoying? Well, me too.
That's why my Netlash collegues and me created the Spamgarden.
Spamgarden is a WordPress plugin that takes random words from your recent comment-spam, and uses them as fertilizer to grow your own private Spamtree."
Check it out at http://www.netlash.com/log/spamgarden
If you would like to see a demo, here's the url: http://www.netlash.com/spamgarden/