[WebAppSec] The idea of negative CAPTCHAs

Spam and automatic submitters really are a problem. One idea to defend this are CAPTCHAs. CAPTCHAs are noisy images and the user (usually) has to recognize the text in the image and enter it in a field. Although some weak algorithms are already broken, this is a good way to keep junk content away. But as automatic recognition gets better, the CAPTCHAs get more sophisticated, and thus harder to read for humans. CAPTCHAs are annoying.
Negative CAPTCHAs
The idea of negative CAPTCHAs is not to ask a user to proof that he's human, but reveal that a spam/login robot is a robot (bot). Most bots are really dumb, they crawl the web and enter their junk in every form's field they can find. Negative CAPTCHAs take advantage of that and include a "honeypot" field in the form which will be hidden from the human user by CSS or JavaScript. Ned Batchelder has several ideas how to do that in his original post.
On the server side, you will check the value of the field: If it contains any text, it must be a bot. Then, you can either ignore the post or return a positive result, but not saving the post to the database. This way the bot will be satisfied and moves on. You can do this with annoying users, as well.
Next step 
This is the basic idea of negative CAPTCHAs, you can make them more sophisticated with Ned's help.