BotBlocker

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:

1. Works out of the box to block spam
2. Automatically stops spam-bots from commenting
3. Zero hindrance to users: no CAPTCHAS, math questions, passwords or extra input required
4. Zero false positives and ignores registered users
5. Can be configured to completely block spam-bots or flag comments as spam
6. Spam detection messages can be easily adjusted
7. No javascript or cookies required
8. Hooks into wp-comments-post.php to prevent spam-bots from directly submitting spam comments
9. Should work fine with most customized comment forms
10. No API keys required

http://www.lform.com/botblocker/ for more information.

Plugin by LFORM, a web design company in New Jersey.