For those not familiar with Mollom, it's a plug-in module compatible with various content management systems. Its main purpose it to act as a sort of safety net between a comment being submitted and the comment actually showing up on the user's site. Before actually being posted, the comment is sent to Mollom's servers for evaluation. Mollom's algorithm rates the comment on if it thinks it is valid or spam, and passes it back to the user's server, which can then decide whether or not to post it. In addition, to its comment spam-blocking features, Mollom also boasts a capability to block the creation of bogus user accounts.
Mollom is currently supported by Drupal, WordPress, and Joomla, as well as the recent additions Radiant (a CMS built on the Ruby On Rails framework) and DaliCMS. Support for other systems is forthcoming.
Mollom seems to be getting better and better and it's nice to see it come out of beta. As strong a product as it is, Mollom has quite a few free and commercial competitors, such as Akismet, Defensio, and TypePad AntiSpam, which also are worth checking out.