In the folder called extras there's a sample plugin called reCaptcha that I installed but couldn't get to work. I've spent two days trying to get this plugin to work, reading all the MT forum posts on the subject, with no luck. It seems that nobody else has been able to get it to work properly either.
Josh Carter has developed a plugin for Movable Type that on it's own didn't work, but in conjunction with the one provided with the release, seems to be working. I had to manually enter the keys into the Recaptcha.pl file, because it wasn't working when the keys were entered on the plugin screen alone. This seems so absurd to me, but there you have it. Don't follow Josh's instructions for adding code to the comments.pm file when installing the plugin. They don't work with this new version of MT.
You need to add the code to the comment-form template for the captcha image to show up, and remove the reference to the div id.
Look for the following line of code:
<div id="comments-open-captcha"> </div>
Remove the id and leave a plain div. Insert the code following between the div elements:
<$MTRecaptchaBox$>
Save and rebuild the site. Captchas should now be working. Good luck.
Update: (Sept 9) Josh has instructions
for getting Movable Type's version of ReCaptcha to work. It seems to
work with a new blog, but didn't appear on this one and I went back to
using both.
Thanks in advance!
download the recaptcha zip from http://multipart-mixed.com/software/recaptcha.html
on lines 62 and 63 you'll see private and public key, enter the keys here. Install both the recaptcha plugins and edit the templates, with the code from multipart. Good luck.