Mollom is a web service that helps you identify content quality and, more importantly, helps you stop spam on your blog, social network or community website. The goal is to make moderation easier, you have more time and energy to interact with your community.
The Mollom service definitely reduces the amount of junk form submissions coming through, however the current (v1.7 at the time of writing) Mollom Drupal module isn't very flexible in what forms in your Drupal powered site it will protect.
All the common forms are covered: user registration, user log in, commenting, node creation. However when you want to protect a custom form on your site there's no documented clean way to add support. With a bit of mollom.module tracing and some test hacking this morning I've come up with a solution to add Mollom analysis to you custom forms using a relatively small module.