
A quote from the creators page:
Mollom is a web service that analyzes the quality of content posted to websites. This includes comments, contact-form messages, blogs, forum posts, etc. Mollom specifically tries to determine whether this content is unwanted - i.e. "spam" - or desirable - i.e. "ham."
This module does a wonderful job in blocking spammers from my site. When someone posts a comment, for example, Mollom sends the data to http://mollom.com, it goes through an analysis, it then returns the content back and classifies it as 'Spam' or 'Ham, now if it is unsure as of what to do with the information, it tags it as 'Unsure', it then presents it to the visitor with a CAPTCHA challenge.
You don't need to know all the details about how it works, just know that it guards your web site against spammers, so you can focus your time on generating content for your blog.
Mollom is a module that was initially written for http://drupal.org, but has now been ported to other CMS platforms. Here is a version for WordPress http://wordpress.org/extend/plugins/wp-mollom/ and for Joomla http://extensions.joomla.org/component/option,com_mtree/task,v...
If you have a Drupal site and you would like to install it, visit http://drupal.org/project/mollom, the main page is at http://mollom.com