drupal captcha module
http://drupal.org/project/captcha 6.x-2.0 on 6.14 (and much earlier versions)
Main function:
Try to outwit spammers by asking a question that humans answer better than robots.
Example function:
Trying to reduce spam on any (and all) forms that users, including spammers, can fill.
To be continued...
Thoughts:
I have the feeling that spammers are using forms not only to post content but also just to send emails. This, to possibly benefit from the rumored search engine practice of scanning emails for statistical puproses. So no wonder spammers would be interested. Let's remember that emails are about just as private as standard postcards, unless they are encrypted.
Experience:
With the math captcha running, spam (that appears automated) still comes through.
I used the image captcha for a long time, and on two manually created sites I still do, because I have no alternative. But the pure image captcha cannot be read by programs used by people with accessibility problems. It should actually not be used.
Maybe it's time to try one of the other module variants. But since a month or so ago, that now requires an additional module (with a lot of various versions). Some of my module lists are getting awfully long.
Trying http://drupal.org/project/riddler. Tried that, and it could be effective and fun for some sites. But I think not for mine.
Tried the spam module http://www.janroe.com/node/75. But this one does not protect the contact form and appears to be responsible for max_allowed_packet errors. Those finally won't go away for larger posts even after substantially increasing the default MAX_ALLOWED_PACKET value.