reCAPTCHA WordPress Plugin

The reCAPTCHA WordPress plugin uses a CAPTCHA to prevent comment spam. Here is how to add reCAPTCHA to your WordPress blog:

Download the zip file.
Unzip the recaptcha folder into your WordPress wp-content/plugins directory.
Activate the plugin on the Options | Plugins Management page of your WordPress admin site. A web form will prompt you to enter a public and private API key. You can sign-up for the keys at the ReCAPTCHA site using the link provided, and then enter them in the text fields to activate the plugin.
That's it! Your reCAPTCHA widget should now appear on the comments page.

FAQ

HELP, I'm still seeing comment spam

There are two common issues that make reCAPTCHA appear to be broken, but are actually not problems.

Moderation emails: reCAPTCHA marks comments as spam, so if you get moderation emails when spam comments are sent, you will get moderation emails for all spam comments with reCAPTCHA. We highly recommend turning off moderation emails with reCAPTCHA.
Trackbacks and Pingbacks: reCAPTCHA can't do anything about pingbacks and trackbacks. You can disable pingbacks and trackbacks in Options | Discussion | Allow link notifications from other Weblogs (pingbacks and trackbacks).

How do I update my reCAPTCHA keys

You can use the WordPress Plugin Management page. On this page, there is a tab for reCAPTCHA options, allowing you to set an API key

reCAPTCHA is missing from the Plugins Managment page

Change the reCAPTCHA directory permissions to 755, but not the contents (which should be 644). E.g. '/.../wp-content/plugins$ chmod 755 recaptcha-wordpress-2.5'