VidoopCAPTCHA is a free web service that determines the difference between
humans creating a new account on your web site and a computer program aiming to
use the account for spamming purposes. Compared to conventional text-based
CAPTCHA, VidoopCAPTCHA is easier on the user by employing images rather than
distorted words to verify if the user is human, and delivers the web site a
marketing advantage through superior usability and branded image placements.
This plugin requires PHP5, compiled with SimpleXML and libcurl.