Google is experimenting with a new type of image oriented CAPTCHA "What's up?" It is basically an image looking on which only a human can decide which side the picture is up. It's easier for human to make a judgment as what is the correct postion and a bit difficult for robots. The report, called "What's Up CAPTCHA?" (.PDF) outlines a new version, which uses image orientation, forcing a user to adjust randomly rotated images to their upright orientation. Although it's pretty premature to judge it's efficiency right now but it's a step in right direction. However RSnake has slightly views on the issue.