Spamicide

The purpose of Spamicide is to prevent spam submission to any form on your Drupal web site. Spamicide adds an input field to each form then hides it with css, when spam bots fill in the field the form is discarded. The field, and matching .css file, are named in such a way as to not let on that it is a spam defeating device, and can be set by admins to almost anything they like(machine readable please). If logging is set, the log will show if and when a particular form has been compromised, and the admin can change the form's field name (and corresponding .css file) to something else.

The install routine sets some default forms as a minimum defense, and admins can turn it off for these, but it's not suggested, it's really the reason it was installed.

Similar Modules

It should play nicely with the CAPTCHA module, but Spamicide doesn't require user interaction.
Spamicide is similar to the Gotcha module, but that only works for contact forms, Spamicide should work on any form.
It should also play nicely with the Spam module.