Yoshimura Hiroyuki

This Plugin blocks comments which is judged to be ‘spam’ by a standard WordPress function. When a ‘spam’ comment is submitted, an error message is shown and the comment will not registerd.
このプラグインは、WordPress標準のコメントブラックリストによる判定の結果’spam’となったコメントを、登録させないで、エラー画面を表示する。
つまり、スパムコメントが溜まりません。
<h3>Arbitrary section 1</h3>

Spam Blocker S1

This Plugin blocks comments which is judged to be 'spam' by a standard WordPress function.

Silkalns

WP Chill

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.

Spam Blocker S1 vs Uber reCaptcha