I've just installed the AuthImage anti-spam plugin for Wordpress. Seems to work a treat. This should cut down on the amount of comment spam I recieve (it was getting pretty annoying). I was hoping it wasn't going to come to this, but it did.
One minor correction to the docs: They say that shell access is required. This is not true. Instead you can check whether your PHP install has the required libraries by creating a file called
info.php containing a single line
<?php phpinfo(); ?>
Visit this page in your browser, and look in the output for GD and Freetype both enabled.
This type of anti-spam protection falls under the category of "captcha" tools, which will be effective until the spammers decide to use legions of pr0n-seekers to solve the captcha challenges. Although this is a legitimate problem with captcha in general, I have doubts about the scalability. There would have to be a LOT of pr0n-seeking drones out there solving captchas for the rate of spam to equal current levels. Also, if it became widely known to these people that their puzzles were being used for spam, presumably at least some of them would refuse to participate? So I'm cautiously optimistic that captcha will be an effective weapon, despite the spam/antispam arms race.