reCAPTCHA in WP comments form plugin is an ANTISPAM tool that adds a Google reCAPTCHA field inside the comments form of your WP theme when the user is not logged in preventing fraudulent or deceptive comments.
The plugin also introduces a second verification process that detects the unauthorized direct accesses by spam robots to the WP comments system and allows you to decide what you want to do with those comments.
Finally, the plugin has got an optional forced javascript output mode that lets you to add a reCAPTCHA field also in old WP themes that didn’t use the new WP form comments functions but they make a direct output of its own comments form.