Mollom is a new initiative launched by Dries Buytaert, the founder of Drupal, and Benjamin Schrauwen, to provide a complete solution to prevent spam and helps improve overall content quality. Mollom provide a set of features including :

• Accurate spam filtering
• Eliminate your moderation queue
• Detailed statistics
• Image and audio CAPTCHA challenges
• Content quality assessment
• and more ...

Tijs Verkoyen have written a PHP5 class to communicate with mollom's open API. Very simple and easy to use. You can find solutions for your languages also available including Java, Ruby, Python, .NET, in addition to a drupal module to integrate Mollom in your community's CMS.