These slides illustrate an idea of mine that I developed further with Kathrin during a course.
It outlines how I would approach the CAPTCHA problem with a collaborative approach (I call it FUNTCHA). Humans provide riddles for other humans, containing of a picture and a question. As in CAPTCHAS, the answer is one word, but the search space to crack FUNTCHAs can be astonishingly high, as it requires general problem solving ability (as humans possess it) to come up with an anwser (if the FUNTCHA is made good).
The problem is less if this would work, but if a large enough community of riddle-loving people can be developed.
Here is the original blog post where I came up with the idea: http://www.nicolashoening.de/?blog&nr=63