You might have seen ‘Enter the characters displayed in the image’ and so would you have seen some audio Captcha.
But some think beyond usual…
Pet Captcha Screenshot (Click to Enlarge)
This captcha is found in Tablefy. (On 21-Feb-2009). Here, we are asked to select all the cat photos to prove human. When you hover over the thumbnail you can see the enlarged image from petfinder site and a link to click, ‘Adopt Me’. These guys should definitely be animal lovers 
So if you are planning to integrate captcha in your site, try some new ideas. (And not one of these )
