Web Directory

» 3DCaptcha
   Open source system that uses spatial cognition abilities of humans to differentiate them from machines.
   http://www-personal.umich.edu/%7emressl/3dcaptcha/

» CAPTCHA: Telling Humans and Computers Apart Automatically
   Includes demonstrations, research papers, and news.
   http://www.captcha.net/

» Captcha Creator
   PHP Captcha Script that generates unique captcha images to stop automated submissions, highly customizable, easy to install and configure.
   http://www.captchacreator.com/

» Inaccessibility of CAPTCHA
   A W3C Working Group Note explaining how CAPTCHAs can present a major obstacle to users who have limited vision or a learning disability such as dyslexia. This document examines a number of potential solutions and alternative systems.
   http://www.w3.org/TR/turingtest/

» JCaptcha
   An open source Java framework for captcha definition and site integration. Project information, documentation, issue tracker, and download.
   http://jcaptcha.sourceforge.net/

» Securimage
   Open source PHP script for generating image CAPTCHAs with audio alternatives. Includes documentation, a gallery and an online demonstration. [LGPL]
   http://www.phpcaptcha.org/

» TheCAPTCHA
   Offers a free and open source PHP script to reduce spam, also contains a detailed article about CAPTCHAs.
   http://www.thecaptcha.com/

» What’s Up CAPTCHA? A CAPTCHA Based On Image Orientation
   Paper by Rich Gossweiler, Maryam Kamvar, and Shumeet Baluja describing an investigation into a new type of CAPTCHA based on asking users to say which way up an image should be.
   http://www.richgossweiler.com/projects/rotcaptcha/rotcaptcha.pdf

» Wikipedia: CAPTCHA
   Explains how captcha features can keep spammers from signing up for certain accounts online, such as email or weblogs. Explains how the system can be circumvented as well.
   http://en.wikipedia.org/wiki/CAPTCHA

» reCAPTCHA
   A captcha system that uses successful decodings to helps digitise books for online use. Background, and details on how to incorporate into a site. Source code in several languages, and plugins for popular blogging and CMS software.
   http://recaptcha.net/