"create a honeypot form field that should be left blank and
then use CSS to hide it from human users, but not bots.
When the form is submitted, you check to make sure the value
of that form field is blank."
I really like this approach - it is much less invasive than the usual word verification. I haven't implemented comments here yet, but I'll probably apply this when I do.
Take a look at the full article here:
http://haacked.com/archive/2007/09/11/honeypot-captcha.aspx
