We have released a new Drupal module to fight spam - Spam Blackhole. This is a simple module to fight automated spam submissions on Drupal sites by bots that are not manually supervised.
The module requires that Javascript is enabled on the site that is running the module. The module works by replacing the form action with a dummy URL while the form is generated from Drupal and then replacing the dummy URL with the actual URL when the user clicks the form submit button.