This site uses affiliate links as a means of monetization.
Mollom is also now available for Drupal, WordPress, Radiant and Silverstripe using plugins and is a very nice project. We've used it in the past and continue to do so now and find it quite capable. According to Dries Buytaerts' blog.. Mollom is now protecting approximately 5000 websites and has an efficiency rating of 99%.
Not bad for something that has only been around for under a year.