Radek Mezulanik

This plugin:
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Add 3 hidden fields to Registration form and to Comments form, that only robots can see. Plus you can choose to add validation checkbox to forms.
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Add to .htaccess file direct access protection for comments file.
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Check time user spent on page and time when user send a form.
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Disable trackbacks for past and future posts.
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Additionally you can choose to add validation checkbox to forms.

Antispam Revolution

Three way antispam protection for registration and comment forms.

Berg Informatik

The plugin searches the content before output and replaces all emails with a tricky javascript spam protection. The visitor will not notice any change, only spam bots can not grab your emails anymore. For your themes, the plugin provides some handy helper functions to protect emails.
<pre><code>BinEmailSpamProtection::cryptMail('mail@example.com') = javascript:DeCryptX('jogpAflti/di')

BinEmailSpamProtection::cryptMailLink('mail@example.com', [optional text]) = <a class="noSpam" href="javascript:DeCryptX('jogpAflti/di')">mail@nospam-example.com</a>

BinEmailSpamProtection::linktext('mail@example.com') = mail@nospam-example.com
</code></pre>
To avoid problems when the plugin is not active, check to see if the class exists before using it:
<pre><code>$protected = (class_exists('BinEmailSpamProtection') ? BinEmailSpamProtection::cryptMail('mail@example.com') : 'mailto:'mail@example.com');
</code></pre>

Email Spam Protection

The Email Spam Protection Plugin is an easy-to-use and configuration-free plug-in to protect email addresses from automatically collecting by spam bot …

Antispam Revolution vs Email Spam Protection