Mollom: Dries' 3rd kid
I've been meaning to upgrade the Akismet module on this site for a while now. Of course, I waited so long that another option just appeared, one I've been waiting to see for a while: Mollom.
The other other project from apparent insomniac Dries Buytaert, Mollom is a content filtering service similar to Akismet. I've actually been familiar with it for a long time, as the GoPHP5 project has been running a Mollom private beta since last June. In fact, when Acquia was announced my first thought was "wait, what happened to Mollom?"