KittenAuth

You've probably heard of CAPTCHAs (the most famous form being the distorted text presented as an image, where you have to enter the word into a form field); KittenAuth is similar but different.

It shows you nine pictures, three of which are kittens. You have to click the three pictures of kittens in order to pass.

It has some weaknesses (e.g. being inaccessible to blind people -- or even sighted people with low vision or in bad lighting, since the resolution is [presumably deliberately] low and pixellated; if you only have a fixed store of pictures, at some point a robot will have seen them all), but seems an interesting idea.