What is Mollom?
When a user posts a comment or other content to your website, your web server sends this content to Mollom for evaluation. Mollom tells your site whether it thinks the content is spam and gives it a quality score. By monitoring content activity on all sites in the Mollom network, Mollom is in a unique position to determine if a post is potentially spam; not only based on the posted content, but also on the past activity and reputation of the poster.

Mollom is actively developed and maintained by Dries Buytaert, the founder and project lead of Drupal, and Benjamin Schrauwen. Mollom is currently used by more than 30,000 sites, including Sony, Adobe, LinuxJournal, Warner Bros Records, Netlog, NBC, and others. As of September, 2010, Mollom currently protects over 30,000 websites with an average efficiency rate of 99.93% - which means that only 7 in 10,000 spam messages are not routinely caught by Mollom's filters.
Mollom website: http://mollom.com

Mollom features
* Next generation spam filtering technology … Ham or spam?
    When new content is analyzed by Mollom's intelligent text-analysis filter, and Mollom is unsure whether it is ham or spam, it asks the user to answer a CAPTCHA challenge.
* Eliminate spam moderation queues;
* Accessible and secure CAPTCHAs;
* Content quality assessment (BETA);
* Multi-lingual.
More features listed on: http://mollom.com/features