A few months ago 3 Google employees explored an innovative approach to CAPTCHA (What’s Up CAPTCHA? A CAPTCHA Based On Image Orientation). Essentially they suggested that having a user orientate an image into its upright position is easier for users (typing difficult to read text can be problematic) and harder for computers.
A computer finds it really difficult to know if this is correctly orientated.
Here is a version I wrote which to can use on your website.
This script makes use of jQuery to allow the user to rotate the images and a little PHP to check the image was rotated into the correct position.