3DCaptcha - http://www-personal.umich.edu/%7emressl/3dcaptcha/
Open source system that uses spatial cognition abilities of humans to differentiate them from machines. |
Captcha Creator - http://www.captchacreator.com/
PHP Captcha Script that generates unique captcha images to stop automated submissions, highly customizable, easy to install and configure. |
CAPTCHA: Telling Humans and Computers Apart Automatically - http://www.captcha.net/
Includes demonstrations, research papers, and news. |
Inaccessibility of CAPTCHA - http://www.w3.org/TR/turingtest/
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. |
JCaptcha - http://jcaptcha.sourceforge.net/
An open source Java framework for captcha definition and site integration. Project information, documentation, issue tracker, and download. |
reCAPTCHA - http://recaptcha.net/
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. |
Securimage - http://www.phpcaptcha.org/
Open source PHP script for generating image CAPTCHAs with audio alternatives. Includes documentation, a gallery and an online demonstration. [LGPL] |
TheCAPTCHA - http://www.thecaptcha.com/
Offers a free and open source PHP script to reduce spam, also contains a detailed article about CAPTCHAs. |
What’s Up CAPTCHA? A CAPTCHA Based On Image Orientation - http://www.richgossweiler.com/projects/rotcaptcha/rotcaptcha.pdf
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. |
Wikipedia: CAPTCHA - http://en.wikipedia.org/wiki/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. |