In a previous post, I talked about the limitations with CAPTCHA systems and proposed a partially-automated turing test to keep non-humans out.
I had a few discussions about this with a friend and he was more interested in the CG Kittens idea I very briefly alluded to in the main text. To summarise, the idea was that if the kittens in KittenAuth were 3D models rendered on the fly, you'd get an essentially unlimited number of images.
Well someone has gone and done something very similar. On a small site called SpamFizzle, there's a description of a 3D CAPTCHA design that renders simple objects and asks the user questions about them. It looks like a great idea to me. It could require some fine-tuning, but I think the premise is sound.
I'd love to credit the author some more, but there are no details on that page. I presume it's the same author as the only other page on the site, Michael G. Kaplan.
Regardless, it's a great idea and worth a read if you're interested.