I came across an interesting page from Microsoft Research. In it, they talk about a new technique for a human interactive proof.
To save you the trouble of looking this up if you don't know, a human interactive proof is a task that humans need to perform to prove that they are not an automated system. An example would be those text boxes that have words that are all distorted and you have to type them into a box, and then click Submit (like when you want to add a new friend on Facebook).
This new technique from Microsoft research borrows from the same idea but adds a new twist: rather than getting the user to type the text into the box, they are given a list of 12 pictures of cats and dogs. The user has to select all of the cats in order to authenticate themselves. This is based on the principle that humans are good at recognizing visual patterns but computers are not (for now). And, given a large enough database (Microsoft has partnered with PetFinder.com which has over 2 million pictures), a brute-force approach doesn't work that well.
I think that this is a rather clever technique. I find the text-box approach difficult to read sometimes but on the other hand, I guess not all the cats look like cats and some people would be prone to missing the occassional furry feline.