Mollom

Mollom is a spam-blocking module for all of your content on a Drupal site, including your comment forms, contact forms, sign-up form, etc. You can set it up to only protect the forms you want, too.

If any of you use Akismet for blocking spam in WordPress, Mollom works the same way: when new content is submitted, the content gets sent to the Mollom servers and checked against their database of known spam, and returns a result to your site, indicating whether or not the content is spam. It goes a little further than Akismet, though (as far as I can tell): if it's not "sure" about whether the content is spam, it displays a CAPTCHA to the user that can be filled out to indicate that the content is legit.