The easiest way to implement an unusual Captcha with images instead of text and drag & drop capabilities.
Captcha is a great thing, it makes it more difficult (hopefully impossible) for robots to submit forms "blindly", creating/sending junk to mails/databases.
However, through time, we've become more and more aware it doesn't work as expected. It difficults communication, and as such, people often make mistakes in the common Captcha forms, making it sometimes a reason for giving up filling a form (specially when the form is big and it doesn't refill the sent information).
So, we thought: "wouldn't it be great to find a way to make a Captcha work better for humans and still be impossible to be submitted automatically by bots?"
We found a great concept for this answer at Ajax Fancy Captcha - jQuery Plugin, but also found in it a few bugs and easy ways for bots to "learn" how to bypass it. As there was nothing really good and innovative as we wanted it, we built our own!
We are concerned about people with accessibility issues. But we also thrive for security.
We are not aware of any other "visual" captchas that have any sort of accessibility, and the "normal" captchas that do, end up compromising security or making it very difficult to understand what is spoken by the scripts (revolving back to the original Captcha problems).
Being innovators as we are, we wanted to implement an accessibility feature that wouldn't compromise security, while making it easy and clear to understand what's spoken by the script. And that's what we did!