Open source system that uses spatial cognition abilities of humans to differentiate them from machines.

PHP Captcha Script that generates unique captcha images to stop automated submissions, highly customizable, easy to install and configure.

Includes demonstrations, research papers, and news.

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.

An open source Java framework for captcha definition and site integration. Project information, documentation, issue tracker, and download.

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.

Open source PHP script for generating image CAPTCHAs with audio alternatives. Includes documentation, a gallery and an online demonstration. [LGPL]

Offers a free and open source PHP script to reduce spam, also contains a detailed article about CAPTCHAs.

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.

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.