Spam on my blog site had gotten pretty bad in the last month. Drupal's spam module wasn't enough to keep it out, and spammers have gotten smart enough to figure out how to do math on the comment forms.
Enter Mollom, which just released to the public not too long ago. Mollom is a third-party blog/website spam filter. This means when someone posts a comment, your site contacts Mollom with that comment for its approval or rejection. Now, they are not the first group to come up with a service like this and won't be the last. The reason I picked up on it is because I've known Dries Buytaert and got to work with him a bit while at the OSL (he is one of the Mollom founders, also the founder of Drupal which I use for this site). So you can say my choice was due to personal trust, not because of any specific feature one over another.
My site is on the very low-end scale of comments. By that I mean that I get maybe one legit comment a week, moreso when I actually post stuff After one week with Mollom I've already noticed a big difference. I haven't seen a single spam slip through, and legit comments are making it fine. In the administration panel you can see a report of spam and ham:
Mollom offers plugins for Drupal, WordPress, Joomla, and Radiant, along with API libraries for most of the popular web languages. Their service is a freemium model, offering free coverage for up to 100 legitimate posts a day (unlimited spam rejections). For more than that you can purchase a subscription. I think this is very generous compared to other freemium models out there in the web2.0 sphere these days, and is plenty enough for most blog and community sites. If you're in need of better spam handling for your site, give them a shot. (I'm not paid to say this, I'm just happy with the service and since it is new I feel there needs to be a bit of review)