From Spam Poison:
WWW Robots (also called wanderers, spiders, crawlers, or bots) are programs that crawl the Web continually retrieving linked pages. When a spammer’s bot visits your website, blog, forum, etc, all pages and sites linked to it will be searched looking for email addresses.
Now you can fight back against their robots!

The links added to your site will redirect email harvesting bots to trap sites that will feed it with an almost infinite loop of dynamically generated fake email addresses, mostly on known spammer owned domains! This will render their harvested lists pratically useless and of no commercial value.

If you are a blogger, add the code from their site to your footer. The code will look like this:

Get it at Spam Poison