by Brandon Fenning on WordPress.org
Kills spam-bots, leaves humans standing. No CAPTCHAS, no math questions, no passwords, just spam blocking that stops spam-bots dead in their tracks.

These are the configuration options available for BotBlocker. While it works out of the box, BotBlocker can be configured in several ways.
Since the vast majority of WordPress spam is automated in nature, the goal of BotBlocker is to block comment spam bots and not hinder users. This is accomplished through the use of honeypot technology, which tricks the spambot into identifying itself by performing actions that a human could not.
BotBlocker’s features include:
wp-comments-post.php to prevent spam-bots from directly submitting spam commentshttp://www.lform.com/botblocker/ for more information.
Plugin by LFORM, a web design company in New Jersey.