Like many other people, I received an email this morning announcing that Mollom, a project of Drupal founder Dries Buytaert, is out of beta. I use Mollom on this website, as well as another personal one, to reduce and sometimes outright eliminate blog comment spam. In my configuration, Mollom works by using heuristics against blog comments and if they are considered spammy enough, the user is challenged to a simple CAPTCHA system to prove that they are not an automated spammer.
Mollom is a great system, since some studies (direct PDF link) have shown that as much as 75% of all attempted comments on blogs are spam!
I've been using Mollom for a few months and have been very happy with it. If you run a blog, even one that isn't in Drupal, I would encourage you to check it out. It's free for many uses, so you lose nothing by at least giving it a shot. There is a developer API and Mollom has already made downloadable integration available for Drupal, WordPress, and Joomla, as well as language-specific libraries.
Congratulations to Dries and the Mollom team! I look forward to the system only getting better as more and more folks start to use it.
Update: A couple folks asked if I'd mind posting my Mollom statistics graph, so you can find it here. There were many hundreds of spams stopped in the past month!