Mollom is a highly effective spam filtering service that can protect almost any web form from spam with a sophisticated image and audio captcha. It's quite useful, in large because it's a centralized service that constantly monitors and learns from spam attacks, making it responsive even when spam bots change their tactics. It also provides an interesting set of results so that you can monitor the spam attacks (and defenses) employed on your website.

While the service is generally found on Drupal-based web sites–one of the principals of Mollom is Drupal's founder Dries Buytaert– it can be used on almost any other system, including Wordpress, Joomla and several others. It can even be used with generic PHP forms.

But while activating Mollom on other CMS systems is generally just a matter of installing a module or plug-in, getting it to work on a in straight PHP form is a little (but not much) more complicated. Here's how to make it work with regular PHP/HTML forms.