We've all had the experience of having to imput some blurry piece of text to prove to a website that we are human. They're called Captchas (Completely Automated Public Turing Test). Sometimes even this human can't tell if that's a "t" or an "f".
Microsoft has teamed up with Petfinder to come up with a new system called Asirra (Animal Species Image Recognition for Restricting Access). What it does, is present you with a series of adoptable animals, and tells you to pick out either the dogs or cats. Easy for us. Very hard for a computer or bot. And if you like one you see, you can click on a button and it will take you straight to Petfinder.
I'm a Mac user myself, but I have to give credit to Microsoft for coming up with a really neat idea.