Securimage is an open-source free PHP Captcha script for generating complex images and CAPTCHA codes to protect forms from spam and abuse. It can be easily added into existing forms on your website to provide protection from spam bots.

It can run on most any webserver as long as you have PHP installed, and GD support within PHP. Securimage does everything from generate complicated CAPTCHA images to making sure the code the user entered was correct.

Requirements: PHP and GD Support
Demo: http://www.phpcaptcha.org/?page_id=12
License: License Free