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.

Daschmi

A lightweight plugin to protect email addresses from email-harvesting. Replace all @ in content with (at) and protect the E-Mail adress with javascript. Inspired by TYPO3.

EmailScrambler

A lightweight plugin to protect email addresses from email-harvesting

Antispam Revolution vs EmailScrambler