Spamgarden

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 https://www.netlash.com/log/spamgarden

If you would like to see a demo, here’s the url: https://www.netlash.com/spamgarden/

Enjoy!