NuCaptcha combines video-animated CAPTCHAs with behavioral analysis to improve the user experience while making it tougher for OCR (Optical Character Recognition) programs to decipher.
Video CAPTCHAs show letters moving back and forth, which could be captured as static images by OCR, but overlapping layers are much harder to recognize.
Behavioral analysis takes into account various cues to assess the security risk of a user. Legitimate users are shown easy-to-solve CAPTCHAs, while suspect spammers are given more difficult ones.
NuCaptcha is not free, details on their website.
VouchSafe uses artificial intelligence to generate challenges based on object associations that humans can recognize but bots can't. For example, drawing a line between two objects that match each other.
Hey, it's harder than it looks! I couldn't figure this one out. Despite the fact I got it wrong, the system could recognize I'm human by the interaction of drawing the line.
One of the tips from the original post was to use a "honeypot field" -- a hidden CSS field invisible to humans, but fed to bots. Anton's comment described a solution that uses 3 buttons with fake input element labels. For example:
“Don’t post this” | “Post this” | “Cancel”
You can catch robots with server-side validation easily because they tend to just "submit."
Ask Bots to Play Nice
Finally, a bonus method. I can't believe I forgot about this one...
Thanks again to our readers! Any more ideas? Please drop us a comment.