Several months ago I added a hidden field to my Email Notification Signup form [1] (and later, renamed the script to flush out any spammers that cached the script location [2]) and boy, was I getting spammed through that script [3].
The thought of using such a techique as a reverse captcha [4] never occured to me. Hiding a form element such that normal users won't see it, but spambots will, and rejecting any request with that field filled in. What a neat idea.
Of course, that's assuming I ever get around to having comments on my blog …
[4] http://bjorkoy.com/past/2007/8/1/battle_comment_spam_with_an/