I don't have a link handy (I've mostly seen these kinds of capchas on small, home-grown websites). It should be trivial to code one up yourself though. Edit: In general, there are a large class of problems that are trivial for humans but would require nearly general AI to be able to solve. Answering the question "What is my username?" is one example. Merely parsing this post to extract the question is challenging, nevermind actually answering it. For a more well-defined set of examples, games of imperfect information (full-ring poker, Starcraft) are still beyond the current state of the art, although I expect starcraft to fall within the next few years.