we all know services (like Hotmail for example) which require to identify the person behind the client to be a human being. I would call it a Turing test, others call it a CAPTCHA (Complete Automated Public Turing test to tell Computers and Humans Apart). We all know those quirky collections of disordered characters to be typed into a textbox. The idea here is that it wont be feasible to use an OCR algorithm to recognize the characters.
Another pretty nice way is asirra (http://research.microsoft.com/asirra/). The idea here is to use a large database of pictures which are totally different beside on fact: They shows either a cat or a dog. Now ask the user to spot all the cats (or dogs, you choose). For humans this is easy (and even fun to do) for machines this is impossible (at least today). By asking the user to spot - say - six cats the change of getting it right simply by randomly clicking on the pictures gets near to zero.
But what do the pets have from being so helpful? The picture database is powered by petfinder.com and you will find a little "Adopt me" button underneath the picture. So while you identify yourself as being a human, you can be a real human and find a friend for live.
PS: Using the service is free and examples are provided.