CheckBot — simple one-click captcha-plugin created for people. It is very easy to use, and look nice, but at the same time, fully customizable, and quite seriously protect your blog from spam.
<h4>Features</h4>
<ul>
<li>Antispam: The plugin uses javascript and randomly generates identifiers and field names, which allows the plugin to automatically filter out fine sent spam.</li>
<li>Support languages: The plugin supports Russian and English languages.</li>
<li>Fully customizable appearance.</li>
<li>The plugin can be used immediately after installation, without additional configuration.</li>
<li>You can configure the output plugin yourself anywhere on the page comments.</li>
<li>If the user is not the correct picture, or even forgot to select it, the plugin will tell him about it, and, just in case it will write a comment.</li>
</ul>
<h3>Help</h3>
You may ask questions at the plugin page – <a href="http://deadblog.ru/checkbot" rel="nofollow ugc">http://deadblog.ru/checkbot/</a>, or write me e-mail – lifeiscoming@gmail.com.

CheckBot

This antispam plugin provides very simple(one-click) and powerful captcha for your blog.

RicheyWeb

HashCash – Proof of Work CAPTCHA

Invisible Proof-of-Work CAPTCHA. No checkboxes, no image grids, no Google, no tracking. Just spam protection that gets out of the way.

CheckBot vs HashCash – Proof of Work CAPTCHA