georf

This plugin uses JavaScript to verify that commentators are persons rather than bots.
If a user’s browser doesn’t support or allow JavaScript, so-called CAPTCHAs are used. The commentator has to enter a fruit name given as image before sending his or her comment.

Obstcha

Decent comment verification to avoid spam without user interaction.

WP Chill

Silkalns

Cristian Raiber

A very useful plugin for everyone using WordPress. Adds reCaptcha security to the WordPress login form, register form and comment form. This plugin could help your blog get rid of a lot of spam comments or brute-force attacks.
Nothing gets passed it if the reCaptcha doesn’t validate.
A few notes about the plugin:
<ul>
<li>Supports audio or image captcha types</li>
<li>Can generate the reCaptcha image / audio type in a number of predefined languages</li>
<li>Adds reCaptcha protection to the WordPress login form</li>
<li>Adds reCaptcha protection to the WordPress register form</li>
<li>Adds reCaptcha protection to the WordPress comment form</li>
<li>Adds reCaptcha protection to the WordPress recover password form</li>
</ul>

Uber reCaptcha

This plugin adds the reCaptcha form to the WordPress login form, recover password form, register form and comment form.

Obstcha vs Uber reCaptcha